RECORD: Daniell, John Frederic. 1823. Meteorological essays and observations. London: Thomas and Charles Underwood.

REVISION HISTORY: Transcribed (single key) by AEL Data 8.2013. RN1

NOTE: This work formed part of the Beagle library. The Beagle Library project has been generously supported by a Singapore Ministry of Education Academic Research Fund Tier 1 grant and Charles Darwin University and the Charles Darwin University Foundation, Northern Territory, Australia.


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METEOROLOGICAL ESSAYS

AND

OBSERVATIONS.

BY

J. FREDERIC DANIELL, F.R.S.

LONDON:

PRINTED FOR THOMAS AND GEORGE UNDERWOOD,
32, FLEET-STREET.

MDCCCXXIII.

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LONDON:

PRINTED BY WILLIAM CLOWES,

Northumberland-court.

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TO

WILLIAM THOMAS BRANDE, ESQ.,

SECRETARY OF THE ROYAL SOCIETY, AND PROFESSOR OF
CHEMISTRY AT THE ROYAL INSTITUTION
OF GREAT BRITAIN,

THESE PAGES ARE INSCRIBED AS A TESTIMONY
OF REGARD AND ESTEEM.

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CONTENTS.

Page
PREFACE ix
An Essay upon the Constitution of the Atmosphere 1
An Essay upon the Construction and Uses of a New Hygrometer 139
An Essay upon the Radiation of Heat in the Atmosphere 207
An Essay upon the Horary Oscillations of the Barometer 251
An Essay upon the Climate of London 268
Meteorological Observations at Madeira, Sierra Leone, Jamaica, and other Stations between the Tropics, by Captain E. SABINE, R.A., F.R.S. 307
Meteorological Observations in Brazil, and on the Equator, by ALEXANDER CALDCLEUGH, Esq. 335
Remarks upon the Barometer and Thermometer, and the Mode of using Meteorological Instruments in general 349
Meteorological Observations upon Heights 376
Meteorological Journal of Three Years 391

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PREFACE.

THE matter of which this volume is composed, has been for a long time accumulating upon my hands, and has, by insensible degrees, increased much beyond my expectations. My attention was first particularly called to the study of Atmospheric Phenomena, by having invented a simple, but accurate, instrument for the measure of atmospheric vapour, which, as complaints of the want of a real hygrometer had been loud and universal, I expected would be well received by meteorologists in general. I was naturally led, in consequence, to commence a series of observations; more with a view of trying the powers of the instrument, than of entering fully upon the general subject. The further, however, I proceeded, the more I became interested; till, at last, I was induced to devote nearly the whole of my leisure time to a pursuit which promised so much of novelty and instruction. After completing three years' observations, at regular periods of the day, besides making numberless other experiments, I thought it time to pause, to see what useful conclusions might be drawn from the previous labour, and what promise

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there might be of advantage in prosecuting the inquiry. I had no wish to continue all my life a mere registrar of the changes of the weather; and I therefore gave up, for a time, the regular observations, and imposed upon myself the more irksome task of calculating and arranging the results which I had already obtained. Having about this period happily become acquainted with Captain Sabine, my ardour was excited by his undertaking to try experiments in meteorology, during the voyage which he was then about to undertake in tropical latitudes. To his friendship I owe the most interesting illustrations of the following pages; and to his conversation, that excitement which has enabled me to complete the work.

The science of meteorology is one of such extent, that its phenomena are probably best studied in detached parts, or monographs; and I have, accordingly, divided my work into separate essays. In the first, I have endeavoured to give a sketch of the general constitution of the atmosphere; and by the developement of a simple idea to unravel the perplexed changes of atmospheric pressure, and to refer to their right cause the oscillations of the barometer. The minor periodical movements of the same instrument form a separate subject of inquiry, as also the radiation of heat in the atmosphere; and such particulars as my own experience has been able to collect of the climate of London, are included in a fourth essay. The instruments of meteorological research have also occupied much of my attention, and I have

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given, in a separate form, full directions for the construction and use of my new hygrometer, together with the necessary Tables. I have also thrown together, in another paper, a few remarks upon the barometer and thermometer. It was my intention to have enlarged more upon the proper method of making Meteorological Observations in general, but it is with the utmost pleasure I find that the subject has been taken up by the most competent authority of the scientific world.

Meteorologists have learnt with much satisfaction, that a committee of the Royal Society has been appointed to take into consideration the state of the society's instruments and the arrangement of the register kept at their apartments, by order of the President and Council. The carelessness exhibited in this department has, for a long time past, been the subject of serious and public complaint; and there is scarcely a person who has had occasion to consult the records, who has not declared them to be unworthy of confidence. Mr. Dalton, Dr. Thomson, and Mr. Howard, have recorded their dissatisfaction; and the latter gentleman has been compelled to ask—" If this learned and highly-respectable body feels the subject of the weather no longer worthy its notice, would it not be better, at once, to dismiss the register from its transactions?" This neglect is the more astonishing as the Meteorological Observations are the only part of the Philosophical Transactions, which are acknowledged

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by the council as a body; responsibility upon all other matters being carefully guarded against by the annually-repeated declaration, that "they do not pretend to answer for the certainty of the facts, or propriety of the reasonings contained in the several papers published, which must still rest on the credit or judgment of their respective authors."

The Meteorological Journal, on the contrary, is always announced as "kept at the apartments of the Royal Society by order of the President and Council," and the injury which may possibly be done to science, by this publication of inaccuracies from authority, can be scarcely appreciated. All foreigners who are concerned in comparing the particular atmospheric phenomena of different regions of the earth, or in the laborious operations of generalizing conclusions from a multiplicity of facts, naturally turn for information upon the climate of London, not to the register of Mr. Howard, or any other private individual, but to that of the Royal Society of London; and what must be their disappointment at finding that their time and labour have been expended upon observations, which, far from keeping pace in accuracy with the progressive improvements of science, have been gradually falling off in common diligence and attention! And what the almost national disgrace which must follow from such a discovery! M. de Luc, in his "Recherches sur l'Atmosphere," makes the following just reflections upon this subject, and concludes

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them with a wish, which those will best appreciate who have ever devoted their time to the extraction of meteorological averages:—

"Il est si commun de substituer l'appareil de l'exactitude à sa réalité, que je tremble qu'on ne vienne tout bouleverser, par des observations mal faites. Combien de savans, dignes de trouver le vrai par la fertilité de leur génie, ne se sont pas exercés dans leur cabinet, à concilier des chimères! Il est donc, à souhaiter, que ceux qui n'ont pas une patience et un dextérité suffisantes, n'apportent rien au dépôt commun."—Tom. iii. p. 310.

Under these circumstances, it is not surprising that the vigour of a new administration of the affairs of the Royal Society should be directed to redress an evil of such magnitude, and to avert reflections so dishonourable.

The names of the members who have been chosen to constitute the committee, the first in the ranks of science, are a pledge that the inquiry will be conducted with ability and perseverance; and there can be no doubt that the result of their labours will be, a full indication of those points in meteorology which most require elucidation; a complete developement of the means and precautions to be used in carrying on an experimental investigation of their nature; and a perfect model of the form of register best adapted to elicit all the advantages of the observations.

The extent of the necessary reform has hitherto

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been little suspected, and, as some prevention of the consequences of the past neglect, ought in candour to be detailed. As conducing particularly to this purpose, it should be generally known that the instruments employed are not such as in the present state of science reflect any credit upon the first philosophic society of the age.

The barometer of the Royal Society has been filled in the common way, without boiling the mercury, or adopting any other means of freeing the tube from that residual air and moisture which is known to adhere, with so much obstinacy, to the surface of the glass. In taking its height, no correction is ever applied for the alteration of level in the mercury of the cistern, or for the change of density in the metal from variations of temperature; and the observations are made without any regard to system. The change in the hours of the summer and winter registers is obviously regulated by nothing but the observer's night-cap, and the whole arrangement is open to the sarcastic remark of the editors of the Annales de Chimie: "L'une des observations est de deux heures après midi, et ne répond, conséquemment, ni au maximum ni au minimum de l'oscillation diurne; l'heure de l'autre change chaque jour puisqu'on la fait au lever du soleil; il serait, je pense, difficile de dire ce qu'un tel choix peut avoir d'avantageux, à moins qu'on ne cite la commodité de l'observateur; mais s'il en était ainsi, je proposerais de faire encore plus pour lui en le dispensant toutàfait de consulter

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le baromètre. A quoi bon publier, en effet, des observations si malordonnés et qui ne peuvent pas même servir à calculer le pression moyenne atmosphérique pour le lieu où elles ont été faites."—An. Chim., vol. vi., p. 441.

Nothing can be more certain than that the barometrical observations of the Royal Society do not, at present, furnish the necessary data for calculating the mean height of the mercurial column.

The thermometers do not appear to have been the subjects of any greater care than the barometer. No verification has been made of their graduation; and they have been supplied without inquiry by the common makers of instruments. Observations, thus made, are totally inapplicable to any nice purpose.

The hygrometer, of De Luc's construction, has been long disordered, and has ceased to occupy a column of the Journal. Upon this occasion I am rejoiced to think that the instrument which I have invented is likely to undergo discussion before so enlightened a tribunal: if there be any merit in its principle or contrivance, it will doubtless be impartially acknowledged.

The society, I believe, have never possessed a vane, but are indebted for their information upon the course of the winds to a neighbouring weather-cock; and the column of the journal which registers the strength of the aërial currents, it will be allowed, is liable to suspicion, when out of 730 observations in the year 1821, 669 are marked with the standard force of 1.

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The dip of the magnetic needle, at London, in 1821, is stated in the Philosophical Transactions to be 71° 6′, or, 71° 42′: it was ascertained to be 70° 03′, in the same year by Captain Sabine.

Observations upon the force of evaporation are totally wanting; and the adjustments of the rain gauge are, I should imagine, unparalleled. Its height is yearly stated, with due precision, to be "114 feet above the level of low water spring tides at Somerset-House, and 75.6 inches above the surrounding ground:" but the Philosophical Transaetions omit to state, what is of infinitely greater consequence, that the funnel is placed immediately under the cowl of a chimney, and that part of the duty of the clerk of the Royal Society is "ever and anon" to pass a wire up and down the pipe to clear it from accumulated soot! No wonder that the quantity of precipitation should vary with the wind, or that the amount of rain should be greater in summer than in winter! To object to any of the particulars of the construction of the instrument after this, or to complain that the water is left to collect for weeks and months before it is measured, would be comparatively insignificant criticism.

These glaring defects the committee have no doubt long since rectified, as no one will believe that they have been suffered to exist, a month after their first meeting. But their other duties must have been of a more arduous nature, as a year has elapsed since their appointment, and no report of their progress has yet appeared. As great bodies are proverbial for the

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slowness of their motions, and as, doubtless, the committee of the Royal Society are anxious that any communication from them upon such a subject should be perfect in all its parts, some time may possibly yet elapse before the scientific world in general are allowed to profit by their labours. I have, therefore, been induced to throw together the few imperfect observations which my daily experience for four years has enabled me to make, for the chance of the little good they may possibly operate in the interval.

The Committee of the Royal Society early in their session did me the honour to request that I would attend to the construction of a new barometer; a commission which I willingly undertook with the anxious wish of forwarding their useful designs, to the utmost of my ability. After a considerable portion of labour and vexation, I had the satisfaction of completing an instrument, with the assistance of Mr. Newman, which I am induced to hope will bear a comparison with the best that has ever been constructed. During its progress, I was led to the adoption of a new process for filling the tube, and made some general remarks which I conceived to be not without interest I was consequently induced to draw up a paper upon the subject, which the press of more important matter has, I presuine, prevented being read, much less published; and, as I conceive, (too partially, perhaps,) that some of the suggestions contained in it may be of practical use, I have taken this opportunity of making them known: In so doing, I cannot be charged with any disrespect

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to the Royal Society, for I have patiently waited their leisure for one whole session.

The valuable observations of my friends, Captain Sabine and Mr. Caldcleugh, in tropical climates, I have given in a separate form, as nearly as possible, in the words of their own memoranda, so obligingly communicated to me; and the details of my own observations conclude the volume. These are the substrata upon which I have founded my theoretical reasoning, and whether I shall be considered to have succeeded in the superstructure or not, they cannot, I conceive, but be deemed valuable accessions of facts and experiments.

In the Essay upon the Construction and Uses of the Hygrometer, I have gone at great length into the subject of its application to the correction of barometrical mensurations, and, I trust that it will be found that I have freed these important operations from the errors and ambiguity arising from atmospheric vapour: in the observations at the end of the volume, I have indicated the probability of other disturbing causes, which are well worthy of further investigation. But such an investigation, as well as the whole subject of meteorology, now requires extensive and careful co-operation, and the detached labours of individuals are utterly incompetent to effect that advancement of the science, which would surely and speedily result from a well-combined plan of observation. In such a general and well-digested scheme, I am acquainted with many who are able and willing to act, and I need

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scarcely say that my own imperfect but zealous exertions should not be wanting. I shall, hereafter, be proud indeed to consider, that any hints derived from the following pages have been found available to so important a purpose. I have endeavoured to obviate some of the objections to which the separation of the subject into essays is liable, by bestowing much care upon the Index, which, I trust, will be found to comprise a complete analysis of the book, and to afford the ready means of connexion and reference.

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AN ESSAY

UPON THE

CONSTITUTION OF THE ATMOSPHERE.

MAN may almost with propriety be said to be a meteorologist by nature: he is naturally placed in such a state of dependance upon the elements, that to watch their vicissitudes and anticipate their disturbances, becomes a necessary portion of the labour to which he is born. The daily tasks of the mariner, the shepherd, and the husbandman, are regulated by meteorological observations; and the obligation of constant attention to the changes of the weather, has endued the most illiterate of the species with a certain degree of prescience of some of its most capricious alterations. Nor, in the more artificial forms of society, does the subject lose any of its universality or interest: much of the tact of experience, indeed, is blunted and lost; but artificial means, derived from science, supply, perhaps inadequately, the deficiency; and the general influence is still felt and acknowledged, though not so accurately appreciated. The generality of this interest is, indeed, so absolute, that the common form of salutation amongst many

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nations is a meteorological wish, and the first introduction between strangers a meteorological observation.

But although the atmospheric phenomena have excited the attention of all classes of men, from the earliest ages of the world; and have probably formed the most ancient and universal theme of conversation and speculation, both with the learned and unlearned; and although they may have been, daily, nay hourly, discussed since the time when the human race were first exposed to their influence; the observations of the vulgar, and the theories of the philosopher, have been alike insufficient for a rational and satisfactory explanation of their general laws. Many and ingenious are the instruments which the science of modern ages has constructed for the accurate appreciation of these perpetual changes; and diligent have been the observers who have dedicated their time to the science of meteorology: but, from the first contrivance of the barometer to the present day, the great and unceasing fluctu-ations of the vast aërial ocean, denoted by that instrument, are unexplained. "The wind bloweth where it listeth, and we cannot tell whence it cometh nor whither it goeth." The complication of the processes, carried on in the immense laboratory of nature, the wide-extended circle of their agency, and the ever-varying results of their compound influences, appear to have been too much for the mind to comprehend as a whole; and the powers of reason have been bewildered in the in-extricabie: labyrinth of causes and effects.

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Being myself deeply interested in these inquiries, and having devoted much of my time and attention to experimenting upon this subject, it occurred to me to consider, that although the science of meteorology, contemplated as a whole, had lately made but little progress towards perfection; yet, that the parts of which it is composed, comprising nearly the whole circle of the natural sciences, had been by no means stationary; but, on the contrary, were making rapid strides towards perfection. The elements of the science, considered as founded upon experiment and observation, have been largely extended and deeply explored; and a rich accumulation of facts have been collected, which only require, perhaps, to be properly adjusted, to enable us to raise the superstructure with security. I reflected that, in the present state of our knowledge, this might probably be done synthetically with the greatest advantage; and that by setting out from a few plain and established principles, and by accurately appreciating their mutual influences, there was a probability of ascending to more complicated relations; till at length, by gradual steps, we might possibly accomplish the explanation of those atmospheric phenomena, the analysis of which has hitherto been perplexed with insurmountable difficulties. This idea has been so strongly impressed upon my mind, that I have resolved to institute such a process, and with this clue, to venture in a path in which so many have failed before me.

Before I proceed, however, to attempt the pro-

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blem which I have contemplated, it may not be improper to prove the necessity of further illustrating a subject which has already exercised the ingenuity of so many and such distinguished philosophers. For this purpose I cannot, I think, do better than refer to the latest hypothesis upon the cause of the rise and fall of the barometric column, which has recently been advanced by Professor Leslie, in the supplement to the Encyclopaedia Britannica*. This distinguished philosopher, previous to offering a solution of the difficulty, passes a sentence upon the attempts of all those who have preceded him in the task, with which the scientific world in general will be disposed to agree.

"Philosophers have eagerly sought to explain the fluctuations of the mercurial column. They have tried every principle that might appear to exert any influence in modifying the local weight of the atmosphere; but their very numerous attempts, it must be confessed, have hitherto been singularly unsuccessful. It was requisite to shew that such causes would not only give results of the kind expected, but were, besides, fully adequate to the production of the phenomena. In most instances, however, either none of those effects could have followed, or they would occur in a very inferior degree and disproportionate extent."

The principal of these attempts he then proceeds to examine; and having shewn their fallacy, pro-

* Ency. Brit., Sup., Article Meteorology, p. 329.

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pounds the following explanation, as hitherto overlooked, and capable of furnishing a satisfactory solution of a great variety of phenomena.

"It is obvious that a horizontal current of air must, from the globular form of the earth, continually deflect from its rectilineal course. But such a deflection being, precisely of the same nature as a centrifugal force, must hence diminish the weight or pressure of the fluid. The only question, is, to determine the amount of that disturbing influence. Though it should appear quite inconsiderable, in the interval of a short space, it may yet accumulate to a very notable quantity, through the wide extent over which the same wind is known to travel: suppose a current to begin to flow from A, Fig. I,

in the direction of a tangent: it will successively bend from a rectilineal track, at the points B, C, D, &c., on the surface of the earth. The particles of the fluid are, therefore, drawn incessantly from their course by the action of gravity. Their vertical pressure is consequently diminished by the force

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spent in producing this deflection. Wherefore, during the prevalence of the wind, the atmospheric column will press with inferior weight at B, than at A, at C than at B, at D than at C; thus gradualfy decreasing through the whole chain. Suppose the ineterals A B, B C, C D, D E, &c., to be each of them a mile, and that the current Teaches the points B, C, D, E, &c., in successive minutes, a celerity which frequently happens; the deflection at B, owing to the curvature of the earth, would be eight inches, or two thirds of a foot; but tie space through Which a body would descend in a minute by the action of gravity is, 60x60 xl6=57800 feet or 86400 times greatter than the deviation from the tangent. Wherefore the atmospheric pressure would, on that hypothesis, be diminished by the 86400th part, for each interval of a mile from A to D. In the space of 288 miles, this diminution would consequently be the 300th part of the incumbent weight; and over an extent of 2880 miles, it would amount to the 30th part. If we assume the very probable estimate, that storms involve the whole region of the clouds, or attain an elevation of near three miles, the diminution of pressure, occasioned by a long series of deflections in the stream would affect one half of the atmosphere. Wherefore a wind which has blown over a track of 2880 miles at the rate of 60 miles an hour, might cause the mercurial column to subside half an inch. If the velocity of the wind were doubled, which is probably the limit of the most tremendous hurricane, the fall of the Barometer would be four limes greater, and amount to two inches."

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Now I conceive, that it will be no very difficult task to shew that the Professor has been as unfortunate as his predecessors, in his proposed solution: and nothing can better illustrate the difficulty of the problem than such a failure. His error, appears to me, to lie in the misapplication of the term horizontal, in the first sentence of the above extract: as there applied, it is made to signify rectilineal, contradistinguished to parallelism to the surface of the globe. Now what power can be supposed to produce such horizontality as this? Mr. Leslie observes, that deflection from it is "of the saine nature as a centrifugal force:" but is it not obvious that it is itself a centrifugal force? And then whence does such an impulse originate? He has not revealed to us the manner in which he supposes the wind, which he employs, to arise, (and this alone is a defect in his theory;) but upon no known principle, I conceive, can its tendency be tangental to the circumference of the earth. But, granting for a moment, the possibility of such a direction, let us suppose a current to begin to flow from A, in the direction of a tangent, and that it is bent from its rectilineal track at the point B by the action of gravity, how is the unknown force to be renewed, so that the wind at B is again to assume a tangental course? But the hypothesis not only supposes this, but further, that it is infinitely renewable; and the effect which is at first scarcely perceptible, is "accumulated by a long series of deflections."

When the foundations of a theory are, as I think, so palpably erroneous, it can scarcely be necessary

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to remark that, the accordance of the phenomena with it, is not so close as has been supposed; otherwise many inexplicable cases might be adduced to prove its insufficiency. One of the strongest of these is, that wind does not always precede a fall of the mercurial column; but, on the contrary, the greatest depressions of the mercury generally precede a wind. Sometimes also great falls are not attended with wind, and sometimes, the mercury has been depressed to leeward of the storm.

It is the more singular that Professor Leslie should have fallen into this error, as in referring in his treatise to the well-known experiment of Mr. Hauksbee, at the beginning of the last century, he refutes its fallacy in a way which is equally applicable to his own hypothesis; if, indeed, it be not the very same idea, clothed in another dress.

"To explain," says he, "the descent of the Barometer during wind, a very ingenious idea has been proposed, which, being apparently confirmed by experiment, has obtained general reception. It is conceived that a current of air, in sweeping over the surface of the earth, must cease to exert any vertical pressure. But this assumption can hardly be reconciled with any strict principle in science, for the particles of air will not for a moment cease to gravitate, nor will any horizontal motion of them produce the slightest derangement in a perpendicular direction." Now the tangental direction of Mr. Leslie's wind, is nothing less than a cessation to gravitate; and its horizontal motion produces derangement in a perpendicular direction, in violation of the very law which he has himself so properly and carefully explained.

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M. Biot, one of the great masters of science, after adverting to the different hypotheses, which have been framed for the explanation of the motions of the Barometer, thus candidly concludes his review of them all.

"Le parti le plus sage est de considérer ces faits comme dès résultats d'observation dont on ne peut jusqu'à présent donner aucune explication Satisfaisante. La hauteur du Baromètre éprouve des élévations et des abaissemens qui paraissent ténir aux modifications de l'atmosphère, mais dont la cause est encore inconnue."—Traité de Physique, Tome I., p. 95.

The method, which has occurred to me, of viewing this complicated subject, promises to simplify the conditions of the acknowledged problem; and I trust to the candour of the learned, to receive the following attempt at its solution with indulgence.

I pass over the particulars of the chemical composition of the atmosphere, as foreign to my present purpose, and consider it as essentially composed of an homogeneous, permanently-elastic fluid, mixed with varying proportions of condensi-ble elastic vapour. I omit, likewise, its relations to light and electricity, and assume that radiant heat passes through it with little or no interruption.

I shall divide the proposed inquiry, thus restricted, into four parts. In the first, I shall consider the habitudes of an atmosphere of perfectly-dry, permanently-elastic fluid, under certain conditions; in the second, those of an atmosphere of pure, aque-

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ous vapour; in the third, the compound relations of a mixture of the two; and in the fourth, I shall endeavour to apply such principles as may legitimately be deduced from the previous investigation, to some of the observed phenomena of the atmosphere of the earth. Many of the observations which I shall have to make, will appear, at first, trite and uninteresting; but let it be remembered that it is from self-evident axioms that the most complicated problems are solved.

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PART I.

On the Habitudes of an Atmosphere of perfectly-dry, permanently-elastic Fluid.

IN proposing the following hypothetical cases, my object has been to assimilate the conditions as much as possible to those of the atmosphere of the earth; separating only the phenomena into classes, that, in considering them singly, we may trace, without confusion, the ultimate effects of each simple cause.

I shall, therefore, propose as the first problem, the natural state of an atmosphere, of perfectly-dry, permanently-elastic fluid, surrounding a sphere in a state of rest, of uniform temperature in all its parts; to the centre of which it gravitates equally?

Its height, density, and elasticity, would every where be equal, at equal elevations; and the column of mercury, which it would support in the Barometer, would be the same every where at the surface of the sphere. These conclusions rest upon the fundamental laws of Hydrostatics, and need no demonstration here. The first condition, therefore, of its state, must be that of perfect equilibrium and rest.

The second condition is, that its density must decrease in a geometrical progression, in ascending through equal stages to its higher regions: for the

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density would every where be proportionate to the superincumbent weight.

The calculation of this progression is simple: for in logarithmic curves when the ordinates are the same, the intercepted portions of the abscissae are proportional to the subtangents, and the process may be conducted in the following manner: We will suppose the length of a column of mercury, at the surface of the sphere, equivalent to the weight of an equal column of the air, to be 30 inches, and its temperature to be 32°; the height of an homogeneous atmosphere, of equal density in all its parts, would therefore be 26250 feet; for the specific gravity of dry air at 32°, and under a pressure equal to 30 inches of mercury, is to that of mercury, as 1 to 10500. From these data, the density for any height may be found by the use of Logarithms*; for as the height of the homogeneous atmosphere, or atmospheric subtangent, is to the height proposed, so is the modulus of the common system of Logarithms, or logarithmic subtangent, to the difference of the Logarithms of the densities. Thus, under the conditions just named, let it be required to know the density or elasticity of the atmosphere at the height of 5000 feet;

Feet. Feet. Modulus. Difference.
then 26250: 5000:: .4342945: .0827227.

which, deducted from .4771212 the Logarithim of 30 inches, leaves .3943985, the Logarithm of 24.797 inches, the density required. The following table represents the decrements of Density, for the different heights subjoined.

* See Young's Natural Philosophy. Vol. I. page 272.

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TABLE I. Shewing the Decrease of Density, and fall of the Barometer at different Heights in an Atmosphere of uniform Temperature throughout.

Height in Feet. Barometer Colmmn inches. Density. Temp.
0 30.000 1.00000 32
5000 24.797 .82656
10000 20.499 .68321
15000 16.941 .53472
20000 14.000 .46677
25000 11.575 .38582
30000 9.567 .31890

The third condition of the atmosphere must be, that its sensible heat shall decrease progressively from below upwards. Experiment has proved that the specific heat of atmospherical air, relative to its mass, increases as the density diminishes: the absolute quantity, therefore, of heat contained in every part of any vertical column remaining unchanged, this gradation of temperature must naturally flow from the enlarged capacity which the air acquires from rarefaction.

The temperature, due to any given height, may easily be found as follows. Reckoning the density of the air at the surface of the sphere = 1, the difference between the density at any given altitude, and its reciprocal being multiplied by 45, will express the mean diminution.

For example, 30,000: 24.797:: 1.000: 826 which is the density at 5000 feet, the density at the surface being 1. Therefore, .826:1.000:: 1.000:1.210,

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and 1.210-826=384x45=17.2, the diminution due to that elevation*. The scale of temperature, appropriate to the preceding heights and densities, is as follows:

TABLE II. Shewing the Decrease of Density and Temperature due to different Elevations in an Atmosphere of permanently-elastic Fluid.

Height in feet. Density. Temp.
0 1.00000 32.
5000 .82656 14.8
10000 .68321 -3.1
15000 .56472 -22.4
20000 .46677 -43.6
25000 .38582 -67.5
30000 .31890 -95.1

We have here another cause developed, which, as well as gravity, affects the constitution of permanently-elastic fluids: namely, an alteration of temperature. A difference of 1 degree, upon Fahrenheit's scale, causes a contraction or expansion of 1/480th part of their volume; which, under equal pressure, proportionally increases or diminishes their specific gravity; or, when confined, raises or depresses their elasticity to the same amount.

From this cause, the barometer alone, will no longer be the exact measure of the progressive density; but for this purpose, its indications must

* See Enoy. Brit., Supplement, Article CLIMATE, p. 188.

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be associated with those of the thermometer. The mercurial column will be shortened 1/480 th. of its length at the several stages, for each degree of depression due to the elevation; and its fall for equal altitudes will differ by that quantity from the geometrical progression. The following table gives the height of the barometer, the specific gravity, and the temperature for the scale of heights before proposed.

TABLE III. Shewing the Fall of the Barometer, at different Heights in an Atmosphere, decreasing in temperature in the preceding Progression.

Height in feet Barometer. Sp. Gravity. Temp.
0 30.000 1.00000 32.
5000 23.949 .82656 14.8
10000 19.106 68321 —3.1
15000 15.229 .56472 —22.4
20000 12.044 .46677 —43.6
25000 9.579 .38582 —67.5
30000 7.566 .31890 —95.1

Such then must be the constitution of an atmosphere of perfectly dry air, surrounding a sphere of the temperature of 32°—perfect equality of pressure producing perfect rest, the specific gravity, pressure and temperature decreasing upwards, according to the above scale, and each being definite for the elevation. These calculations have been made upon the supposition of equal gravity at all heights; a supposition, which is not exactly accordant with fact: the difference, however, is ex-

[page] 16

tretnely small and wholly unimportant to the general argument.

If the temperature of the sphere be now conceived to rise generally and equally in all its parts, a new adjustment of the gaseous strata must ensue. An increase of elasticity will take place, and the total height will be increased. The expansion must necessarily proceed from below upwards; for the impulse, being equal and simultaneous in each of the columns, into which we may suppose the atmosphere divided, they mutually confine one another in every other direction.

As there is no increase or decrease of ponderable matter in any of the vertical sections, the total pressure will remain as before, and the barometer at their bases will not be affected: but as a different distribution of the weight in the different horizontal sections takes place, its height will be altered in every other situation.

Let a b represent a column of fluid, whose total weight is 30, and whose four sections, taken at equal altitudes, are each 7.5: the scale of the progressive heights will be =30.= 22.5=15. = 7.5. Let c d be the same column expanded by ¼ its length, Its total weight will be 30 as before; but the weights of its sections, taken at the same altitudes, will be reduced to 6, and the same progressive heights will be increased to 30. =5 24 = 18 = and 12. The quantity of

[page] 17

matter remains the same, but a greater proportion of it is distributed in the upper parts. But if, in proportion as the expansion takes place, the fluid should over-flow at its upper surface, so that the length of the column may remain the same, then would its total weight from e to d be reduced to 24, and that of its several progressive heights to 18=12 and 6.

The first case presents us with an analogy adapted to our present purpose, the second we shall have occasion to apply as we proceed.

Upon the supposition which has been made, viz., of a general rise in the temperature of the sphere, the temperature of the atmosphere will be raised throughout its mass by internal circulation; but the proportion due to the several degrees of elevation will be preserved. The following Table exhibits the arrangement which would take place from an increase of 16 degrees of heat in the sphere:

TABLE IV. Shewing the effect upon the Barometer of a general Increase of Temperature in the Atmosphere.

Height in feet. Barometer. Sp. Gravity. Temp.
0 30.000 .96668 48
5000 24.072 .80402 31.4
10000 19.338 .66878 14.1
15000 15.525 .55629 —4.3
20000 12.409 .46273 —24.5
25000 9.915 .38489 —47
30000 7.852 .32016 —62.3

The height of the barometer at the base of the column, which denotes its total weight, remains the

c

[page] 18

same as in Table III., but increases at the various stages of altitude: the force of expansion having effected a different distribution of the ponderable matter, and raised a greater proportion to the upper regions.

Through this succession of changes, the atmosphere again attains a state of equilibrium and repose; and the action being equal all over the sphere, the adjustment is soon effected.

Let us next suppose that the temperature of the sphere, round which the atmosphere is diffused, instead of being equal in all its parts, increases by equal degrees from the Poles to the Equator; and let us assume that the temperature of the former is 0°, and the temperature of the latter 80°. The height of the barometer is still to be taken as 30.000 indies upon all parts of the surface. The following Table will exhibit the pressure, density, and temperature, at the two extreme points of such an arrangement, together with their gradual diminution for equal ascents:

TABLE V. Shewing the comparative Densities and Elasticities of two Columns of Air, of different Temperatures, at different Elevations.

Height in feet. Barometric Column. Specific Gravity. Temperature.
Poles. Equator. Poles. Equator. Poles. Equator.
0 30.000 30.000 1.06666 0.90000 0 80
5000 23.597 24.342 .86935 .75737 –18.5 64.4
10000 18.587 19.779 .70856 .63735 –37.8 48.4
15000 14.591 16.060 .57752 .53640 –58.8 31.4
20000 11.411 13.043 .47071 .45150 –82.1 12.8
25000 8.900 10.521 .38365 .37980 –109.1 –7.6
30000 6.906 8.483 .31270 .31980 –140.3 –30.7

[page] 19

In considering this arrangement, we may remark, first, that at the surface of the sphere, the elasticity of the air, as measured by the Barometer, remaining the same, its specific gravity is very much greater at the poles than at the equator; and hence it is clear, that the atmospheric column must be proportionately shorter at the former, than at the latter point.

The further conclusion follows, that this heavier fluid must, by the laws of Hydrostatics, press upon and displace the lighter; and a current will be established from the poles to the equator.

Our second remark is, that this difference of gravity becomes less as we ascend from the surface, and at a certain point is neutralized: while, on the other hand, the elasticity, which is equal at the surface, varies with the height; and the Barometer stands higher, at equal elevations, in the equatorial than in the polar column. This disproportion increases with the elevation; and at some definite height, must more than compensate the unequal density of the lower strata, and occasion a counter-flux from the equator to the poles.

It will be convenient to consider these differences of gravity, and elasticity, as distinct and antagonist powers; and to measure their forces, if possible, upon the same scale. This may readily be done, by considering, that the pressures of equal columns are as their specific gravities, that is to say, .90000: 1.06666:: 30.000: 35.553, which gives an excess of 5.553 inches of mercury, as the measure of the excess of gravity in the case proposed.

This excess of gravity, we have seen, is unopposed at the surface of the sphere by any excess

C 2

[page] 20

of elasticity; so that it is the exact measure of the force with which a polar atmosphere would press upon an equatorial, supposing the two in juxta-position. It is also the measure of the pressure which would be required at the equator to equalize its density with that of the poles. If we could imagine this by any means effected, the Barometer at the former station must rise to 35.55; and the current would be reversed, and flow with the same force from the equator to the poles; this current being now occasioned by excess of elasticity, as it was before caused by excess of gravity. An increased pressure of 2.77 inches would produce a state of perfect repose on the surface; the resulting augmentation of elasticity and gravity being jointly equal to the former excess of gravity. These forces being reciprocal in their action, whatever mechanical cause acts upon the one, must equally affect the other. The following Table exhibits the excess of the two powers, together with their balance for the heights before assumed.

TABLE VI. Shewing the Force of the Polar and Equitorial Currents, at different Elevations.

Height in feet. Elasticity. Density. Balance.
0 - 0.00 + 5.55 + 5.55
5000 - 0.74 + 3.73 + 2.99 Lower Carrent from the Poles to the Equator.
10000 - 1.19 + 2.38 + 1.19
15000 - 1.47 + 1.37 - 0.10
20000 - 1.63 + 0.64 - 0.99
25000 - 1.62 + 0.12 - 1.50 Upper Current from the Equator the Poles.
30000 - 1.58 - 0.20 - 1.78

[page] 21

The lower, or polar current, upon this supposition, extends to the height of about two miles and a half, gradually diminishing in force; and at that height, gives place to an upper or equatorial current, which increases in strength the higher we ascend.

The velocities of these currents may also easily be calculated: for as the velocity of air rushing into a vacuum*, is found by multiplying the square root of the height of the homogeneous atmosphere, expressed in feet, by 8, so will their rate be found, in the number of feet per second, by multipying, by 8, the square root of the height of a column of air, equivalent to their respective forces. Thus the force of the polar current being 5.55 inches, the height of an equipondrant column of air would be 4856 feet, and ✓ 4856 × 8 would give, in round numbers, 557 feet per second, or 379 ¾ miles per hour. The rate of the equatorial current, at the height of 30000 feet, by a similar calculation, would be about 350 feet per second.

But our hypothesis assumes, not that the equatorial and polar columns are in contact, but that the temperature graduates equally between the points; and, if we divide the hemisphere into bands of 10 degrees each, the pressure of the first upon the second will be equal to the second upon the third, and so on; that is to say, the greatest force of the lower current will be, by our calculation, 0.617 inches, and that of the upper 0.240 inches, for every 10 degrees of latitude. Their respective velocities, equalizing them for each degree of heat, may also

* Young's Natural Philosophy, Vol. I., page 279.

[page] 22

be approximated as follows. The extreme differences of 80 degrees, we have seen, are equivalent to 5.55 inches and 2.16 inches; and the intermediate differences of about 9 degrees to 0.617 inches, and 0.240 inches. The differences for each degree will, therefore, be .068 inches and .026 inches, which give a velocity of 61 feet per second for the lower current, and 38 feet for the higher; or about 41 and 25 miles per hour.

This interchange of the polar and equatorial atmospheres, must tend towards an equalization of temperature; and, in fact, would, in time, produce an equal diffusion of the heat of the sphere itself, were not a cause included in our supposition, to provide for the permanency of its existing state.

As we have calculated that these currents, with such respective degrees of force, are the consequence of the equal height of the Barometer all over the surface of our sphere, so we conclude, that this equal height is maintained by this constant and regular flow; and any irregularity communicated to the currents would immediately be shewn by a change in the mercurial column. Let us imagine, for an instant, that any cause (no matter at present whence originating) should retard the velocity of the polar current, without at first affecting the equatorial, it is obvious that the Barometer would fall at the equator, and rise at the poles; for the balance of forces would be disturbed by the want of compensation for the matter removed at one extremity and accumulated at the other.

The subject may derive some illustration from the following figure:—

[page] 23

Let a b c represent a bent tube, in which a column of oil in one leg is balanced by a column of water in the other. The former will be longer than the latter, and their surfaces will stand respectively at e and d. Now a current of oil will pass along the upper part of the level portion of the tube f g, and will ascend through the water in the leg a, while a current of water will occupy the lower part of the level, and these will continue to flow till the lower part of the tube f g be filled with the heavier fluid. During no part of this interchange of places will any alteration arise in the equal pressure

[page] 24

of the columns; for the particles, notwithstanding their horizontal motion, never for an instant cease to gravitate: the longer, indeed, will be shortened, and the shorter lengthened; but their total weights will at all times remain the same. If we could by any means devise a method of retarding the motion of the water, while that of the oil continued the same, the latter would soon flow from the leg b, and be accumulated upon the top of the water in the leg a; and the weight of the column in a would be as much increased as that of the column in b would be diminished.

Thus, then, our first postulatum constitutes an atmosphere which is necessarily at rest in all its parts: our second, occasions one as necessarily in continual and equal motion—let our third be a sphere increasing in heat, unequally, from the poles to the equator. The extremes are to be 0° and 80° as before, and at the middle point, or the latitude of 45, we will suppose the temperature to be the exact mean of 40°-: but from that centre, the increase towards the equator, is to be by a rapidly and equally decreasing rate, for equal distances; and the decrease towards the poles, by a similar progression. The temperatures, for every 10 degrees of latitude, from the poles to the equator, may then be as follows:

Pole Lat. 80 Lat, 70 Lat. 60 Lat. 50 Lat. 45
0 3. 2 9. 6 19. 2 32. 40
Lat. 40 Lat. 30 Lat. 20 Lat. 10 Equator
48 60. 8 70. 4 76. 8 80.

The height of the Barometer is still to be 30.000 inches every where upon the surface.

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TABLE VII. Shewing the Elasticity, spectra

Height. Poles Latitude 80 itu
Feet. Elast. S. Grav Temp. Elast. S. Grav. Temp. Ela Gra
0 30. 000 1.06666 0 30.000 1.6038 3.2 30.0468
5000 23.597 .8635 – 18.5 23.652 .86542 – 15.2 23.7568
10000 18.587 .70856 – 37.8 18.630 .70637 – 34.3 18.7014
15000 14.591 .57752 – 58.8 14.642 .57654 – 55.1 14.7774
20000 11.411 .47071 – 82.1 11.484 .47057 – 78.2 11.6169
25000 8.900 .38365 – 109.1 8.965 .38408 – 104.7 9.1084
30000 6.906 .31270 – 140.3 6.978 .31352 – 135.7 7.1014

TABLE VIII. S

Height. Latitudes 90 & 80 Latitudes 80 & 70 Latitudes 70 &
Feet. Elait. S. Grav. Bal. Elast. S.Grav. Bal. Elast. S. Grav.
0 -- + 178 +.178 -- +.387 +.387 -- + .575
5000 -.055 +.112 + .057 -.055 +.246 +.191 -.086 + .367 + 8
10000 -.043 + .062 +.019 -.094 +.142 +.048 -.169 +.214 45
15000 -.051 +.028 -.023 -.133 +.070 -.063 -.194 +.101 93
20000 -.073 + .004 -.069 -.133 +.021 -.112 -.210 + 025 85
25000 -.065 -.013 -.078 -.137 -.015 -.152 -.212 -.028 40
30000 -.072 -.023 -.095 -.122 -.039 -.161 -.202 -.062 64

[page] 25

Table VII. furnishes us with the elasticity, specific gravity, and temperature of such an atmosphere, calculated upon these data, for every 10 degrees of latitude, from the surface, by equal altitudes, to the height of 30000 feet. Table VIII. exhibits the excess of the lateral pressure of each column upon that which adjoins, arising from the balance of forces.

It will be observed, that the currents still set as before, and at nearly the same altitudes, but with unequal velocities in different parts of their courses. The pressure, the density, the temperature, and the velocity, are all definite, for the latitude, and for the elevation; and it is by the exact balance alone of these circumstances, that the Barometer is maintained at an unvarying height, at the surface of the sphere.

We may also remark, that a change of temperature, which equally pervades a column of air throughout its length, may effect an adjustment of density without disturbing the equiponderant mercurial column situated at its base: the force, however, of the compensating currents will be altered; and, under some circumstances, their courses even may be changed. Let us imagine that the temperature of latitude 50, as it stands in the preceding Table, is altered by some cause not affecting the neighbouring columns; and that the temperature rises from 32° to 60°.8 at its base, and equally pervades its whole length: the force of the current will be increased from latitude 60 to 50, in its original direction, while that from 50 to 40 will be reversed; as will appear more clearly from

[page] 26

the following Tables, in which the change is made according to this assumption.

TABLE IX. Shewing the Alteration of Specific Gravity and Elasticity in an Atmospheric Column, from an Increase of Temperature at the Surface of the Sphere in a given Latitude.

Latitude 60 Latitude 50 Latitude 40
Height. Elasticity. S. Gravity. Temp. Elasticity. S. Gravity. Temp. Elasticity. S. Gravity. Temp.
0 30.000 1.02707 19.2 30.000 .93960 60.8 30.000 .96668 48.
5000 23.793 .84427 1.5 24.215 .78533 44.6 24.072 .80402 31.4
10000 23.793 .84427 – 16.9 24.215 .78533 27.9 19.338 .66878 14.1
15000 14.969 .57061 – 16.9 15.739 .54863 10.0 15.525 .55629 – 4.3
20000 11.827 .46904 – 58.8 12.673 .45856 – 9.4 12.409 .46273 – 24.5
25000 9.314 .38558 – 83.8 10.162 .38827 – 31.2 9.915 .38489 – 47.
30000 7.302 .31699 – 112.7 8.135 .32035 – 55.9 7.852 .32016 – 62.3

[page] 27

TABLE X. Shewing the Alteration of Direction and Force in the Atmospheric Currents, from the same Cause.

Latitudes 60 and 50 Latitudes 40 and 50
Height. Elasticity. Sp. Gravity. Balance. Elasticity. Sp. Gravity. Balance.
0 + 2.560 + 2.560 +0.840 +0.540
5000 -0.422 +1.722 +1.300 -0.143 +0.580 +0.437
10000 -0.638 +1.100 +0.462 0.193 0.380 0.187
15000 -0.770 +0.710 -0.060 -0.214 +0.240 +0.026
20000 -0.846 +0.300 -0.546 -0.264 +0.130 -0.134
25000 -0.848 +0.070 -0.778 -0.247 -0.050 -0.197
30000 -0.833 -0.009 -0.842 -0.283 -0.006 -0289

Here we perceive that the wind, which had blown on the surface from latitude 60 to 50 with a force of 0.810 inches, is now increased to 2.560 inches, and that which set from latitude 50 to 40, with a force of 1.034 inches, now blows from latitude 40 to 50 with a force of 0.840. A corresponding change of velocity and direction ensues in the upper currents, and the compensation of pressure thus takes place.

From the nature and essential properties of a permanently-elastic fluid, it follows that any cause tending to diminish gradually its specific gravity at the base of a column, or to augment it at its summit, must affect it throughout its length; so that, if its heat be slowly increased below, its temperature must rise from one extremity to the other.

[page] 28

But although such a change may take place, as has just been demonstrated, without affecting the length of the equiponderant column of mercury situated at the lower extremity, the barometer will rise at all higher stations. By comparing together Tables VII and IX., this effect will be easily appreciated. The augmentation of temperature in latitude 50 from 32° to 60°.8 takes place on the surface, while the mercurial column remains at 30 inches; but at the height of 5000 feet it rises from 23.949 inches to 24.215 inches, making a difference of 0.266 inches. This difference increases to a certain extent with the elevation. Corresponding changes for less alterations of heat are readily perceived by comparing together the different latitudes of Table VII.

The cases which have hitherto been proposed have all been of the same nature: the alterations of temperature have been imagined to take place in the sphere itself, and from it to have been slowly communicated to the atmosphere, through which they have spread under the regular modifications due to the increasing capacities of its successive strata. Let us next consider the effect which would be produced by the heating of any of the upper layers, from some temporary cause not originating in, or extending to, the lower. For this purpose, in the column appropriate to latitude 30, in Table VII., at the fifth station, or the height of 20,000 feet, we will suppose an increase of heat to take place of 10 degrees. This increase will extend upwards, but the inferior portions remain of their original temperature. Now, the first effect of this change will be, an augmentation of elasticity in the upper beds of

[page] 29

the atmosphere; which, exerting its force upon the high equatorial current, will accelerate its due velocity on one side, and retard it on the other. The expanding air, not being laterally confined by a proportionate expansion of the neighbouring sections, will not accumulate above; but will flow off, and cease its vertical pressure upon that column. The upper regions will therefore be rarefied, and become lighter, and pressing with less weight upon the lower, the barometer will fall at the surface of the sphere in proportion to the amount of the expansion. This effect has been already explained in the second application of Fig. 2.

Let us illustrate this action, by first representing the column, so partially changed in temperature, upon the supposition that no such compensation takes place.

TABLE XI.—Shewing that a partial Alteration of Temperature and Specific Gravity in an Atmospheric Column must affect the Density generally by mechanical Adjustment.

Height. Elasticity. Sp. Gravity. Temperature.
Regular. Irregular.
0 30.000 .93960 60.8 60.8
5000 24.215 .78533 44.6 44.6
10000 19.531 .65639 27.9 27.9
15000 15.739 *.53710 10.0 *20.
20000 12.673 *.44890 — 9.4 *0.6
25000 10.162 *.37520 — 31.2 -*21.2
30000 8.135 *.31360 —55.9 -*45.9

[page] 30

That such a succession of densities would result is certain; for they are due to the given pressures and temperatures: and it is also certain that such a succession could not exist in nature; for it is contrary to the fundamental law of geometrical progression. But if we suppose the Barometer to fall, as represented in the following Table, the regular series is maintained.

TABLE XII.—Shewing the Fall of the Barometer, which would be occasioned by a partial Alteration of Temperature in the upper part of an Atmospheric Column.

Latitude 10
Height. Elasticity. Sp. Gravity. Temperature.
Irregular. Regular.
0 *29.87 .91990 60.8 60.8
5000 *23.70 .76890 44.6 44.6
10000 *19.12 .64270 27.9 27.9
15000 15.73 .53710 *20 10.0
20000 12.67 .44890 *0.6 — 9.4
25000 10.16 .37520 —*21.2 —31.2
30000 8.13 .31360 —*45.9 —55.9

The density of an elastic fluid is the result of its gravity acting upon its elasticity, and by the reaction of these two powers any change in the vertical column is instantaneously communicated throughout its entire length, and no inequality of density can for a moment exist.

Let us now imagine that the local accession of heat, instead of pervading at once the whole of either horizontal section, commences at some de-

[page] 31

finite point, and gradually extends itself in depth. To render the march of this effect intelligible, we will consider its operation at several stages of its progress. We will first endeavonr to appreciate the influence of an increase of 5 degrees of heat, at the height of 5000 feet in the same column of latitude 30. The disturbing cause now affects the lower current, and the expanding air, not being checked by a simultaneous increase of elasticity in the adjoining columns, rushes forwards with accelerated velocity. The diminution of density occasioned by the excessive drain, is distributed throughout the column by mechanical adjustment. The results are as follow:

TABLE XIII.—Shewing the Effect upon the Barometer of a partial Increase of Temperature at an elevation of 5000 Feet.

Latitude 30.
Height. Elasticity. Sp. Gravity. Temperature.
Irregular. Regular.
0 *29.6S .92973 60.8 60.8
5000 24.21 .77713 *49.6 44.6
10000 *19.32 .64957 27.9 27.9
15000 *15.57 .54829 10.0 10.0
20000 *12.53 .45373 -9.4 -9.4
25000 *10.05 .37921 -31.2 -31.2
30000 *8.05 .31690 -55.2 -55.2

The gradual extension of the same increase of

[page] 32

heat to the height of 10000 feet, produces the following arrangement:

TABLE XIV.—Shewing the Effect upon the Barometer of an Extension of the Increase of Temperature to 10000 Feet.

Latitude 30
Height. Elasticity. Sp. Gravity. Temperature.
Irregular. Regular.
0 *29.37 .91990 60.8 60.8
5000 *23.95 .76890 *49.6 44.6
10000 19.32 .64270 *32.9 27.9
15000 *15.41 .53710 10. 10.
20000 *12.40 .44890 -9.4 -9.4
25000 *9.94 .37520 -31.2 31.2
30000 *7.96 .31360 -55.9 -55.9

Thus it appears that the fall of the Barometer would be proportionate to the extent to which the rise of temperature would reach in this progressive manner. A small increase, thus operating, produces the same amount of depression, as if a greater expansion had been exerted in a more limited space. The following Table presents the effect upon the Barometer of a gradual rise of two degrees of temperature, from 5000 to 25,000 feet.

[page] 33

TABLE XV. Shewing the Effect upon the Barometer of a small partial Increase of Temperature gradually extending itself throughout the Column.

Latitate 30, 1st Change. Latitude 30, 2d Change. Latitate 30, 3d Change. Latitate 30, 4th Change. Latitate 30, 5th Change.
Height. Elasticity S. Grav Temp. Elasticity S. Grav. Temp. Elasticity S. Grav. Temp. Elasticity. S. Grav. Temp. Elasticity. S. Grav. Temp.
0 *20.87 .9355 60.8 *29.74 .9314 60.8 *29.61 .9273 60.8 *29.49 .9232 60.8 *29.37 .9192 60.8
5000 24.21 .7819 *46.6 *24.11 .7785 *46.6 24.91 .7751 *46.6 *23.91 .7717 *46.6 *23.81 .7683 *46.6
10000 *19.45 .6546 27.9 19.45 .6518 *29.9 *19.37 .6490 *20.9 *19.20 *6462 *29.9 *19.21 .6434 *.29.9
15000 *15.68 .5463 10 *15.62 .5440 10. 15.62 .5417 *12. *15.56 .5394 *12. *15.50 .5371 *12.
20000 *12.62 .4565 -9.4 *12.57 .4545 -9.4 *12.52 .4525 - 9.4 12.52 .4505 *-7.4 *12.47 .4485 *-7.4
25000 *10. 12 .3816 31.2 *10.08 .3800 -31.2 *10.05 .3784 -31.2 *10.01 .3768 -31.2 10.01 .3752 *-29.2
30000 *8.10 .3190 -55.9 *8.07 .3177 -55.9 *8.04 .3164 -55.9 8.01 .3151 -55.9 *7.98 .3138 55.9

In the full effect of these three examples, represented in Tables XII., XIV., and the last column of Table XV., the progression of density is the same; and the barometer falls to tha same amount at the base of the column.

D

[page] 34

The limit of the action of the irregular expansion of aërial column is fixed, not only by its amount and progress, but also by the compensating effects of the contiguous sections. Its influence upon the laterial currents is calculated below.

TABLE XVI. Shewing the Effect of the preceding Changes upon the Force and Direction of the Currents.

First Modification. Second Modification. Third Modification.
Latitudes 40 & 30 Latitudes 20 & 30 Latitudes 40 & 30 Latitudes 20 & 30 Latitudes 40 & 30 Latitudes 20 & 30
Height Elast. S. Grav Bal. Elast. S.Grav. Bal. Elast. S. Grav Bal. Elast. S. Grav Bal. Elast. S. Grav Bal. Elast. S. Grav Bal.
0 +.63 +1.44 +2.07 +0.63 0. +0.63 +.63 +1.44 +2.07 +.63 0 +.63 +.63 +1.44 +2.07 +.63 0 0.63
5000 +.37 +1.09 +1.46 +0.58 +0.08 0.66 +.12 +1.09 +1.21 +.73 +0.08 +0.81 +.26 +1.09 +1.35 +.47 +0.08 +0.55
10000 +.21 0.81 +1.02 +0.55 +0.140 +0.69 +.0 +0.81 +0.81 +.35 +0.14 +0.49 +.12 +0.81 +0.93 +.46 +0.14 +0.60
15000 -.12 +0.60 +0.39 +0.16 +0.18 0.34 +.11 +0.60 +0.71 +.48 +0.18 +0.66 +.02 +0.60 +0.62 +.39 +0.18 +0.57
20000 -.26 +0.43 +0.17 +0.14 +0.20 +0.34 +.0 +0.43 +0.43 +.41 +0.20 0.61 -.06 +0.43 +0.37 +.31 0.20 +0.54
25000 -.25 +0.30 +0.05 +0.18 +0.21 +0.39 -.03 +0.30 +0.27 +.40 +0.21 +0.61 -.10 +0.30 +0.20 +.33 +0.21 +0.54
30000 -.28 +0.21 -0.07 +0.18 +0.22 +0.39 -.11 +0.21 +0.10 +.35 +0.22 +0.57 -.13 0.21 0.08 +33 +0.22 +0.55

[page] 35

From Latitude 40 to 30, it will be observed, that the force of the polar current is greatly increased; while from 30 to 20, it is reversed. The different modifications of the heating process produce different adaptations of the upper currents, which the comparison of the several tables will sufficiently explain.

It may readily be imagined, that irregularities thus introduced into these compensating movements, the consequence of diminished mechanical pressure, must of themselves be liable to produce changes of temperature in the columns, foreign to the natural gradation; and that, amongst others, the atmosphere, in its upper parts, may be liable to greater depression of heat than would be due to the elevation alone. A gradual process of cooling taking place in the higher portions of a body of air, would communicate itself to the whole mass, in an analogous manner to the equal diffusion which would ensue from the slow communication of heat to the lower parts; that is to say, without producing any effect upon the Barometer, at the surface of the sphere, or any irregularity in the gradation of temperature. But where the change is effected suddenly, by the admixture of a large body of cold air, a mechanical effect is produced by the increased pressure of the mass; and the equilibrium of density takes place before the proper adjustment of temperature. An atmosphere hence results, whose heat decreases in a proportion greater than is due to the decrease of density. The effect is analogous to that which arises from an irregular increase; and the Barometer must rise to equalize

D 2

[page] 36

the specific gravity. The following Table has been constructed upon the supposition of a depression of temperature of ten degrees, taking place in the column of air at latitude 30, and extending at once from the height of 1000 feet to 15000.

TABLE XVII. Shewing the Rise of the Barometer, occasioned by the sudden partial Reduction of Temperature in an Atmospheric Column.

Latitude 30.
Height Elasticity. Sp. Gravity. Temperature.
Irregular. Regular.
0 *30.626 .95917 60.8 60.8
5000 *24.715 .80166 44.6 44.6
10000 *24.715 .67007 *17.9 27.9
15000 15.789 .56003 *0. 10.
20000 *12.933 .46805 -9.4 -9.4
25000 *10.371 .39127 -31.2 -31.2
30000 *8.305 .82697 -55.9 -55.9

The corresponding effects upon the upper and lower currents are deduced below.

TABLE XVIII. Shewing the Effect upon the Currents of the preceding Change.

Latitudes 30 and 40 Latitudes 30 and 20
Height. Elasticity. Sp. Gravity. Balance. Elasticity. Sp. Gravity. Balance.
0 +0.626 +0.234 +0.392 +0.626 +1.284 +1.910
5000 +0.643 -0.075 +0.568 +0.336 +0.989 +1.325
10000 +0.193 +0.041 +0.234 -0.144 +0.765 +0.621
15000 +0.214 +0.120 +0.334 -0.159 +0.568 +0.402
20000 +0.524 +0.172 +0.696 +0.122 +0.419 +0.541
25000 +0.456 +0.207 +0.449 +0.29 +0.313 +0.342
30000 +0.453 +0.223 0.676 -0.008 +0.224 +0.216

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It is not required here to point out all the means by which such changes of heat as we have represented may be effected; or to trace further the endless modifications of densities and currents which would result from their different applications: it is sufficient, at present, to have shewn that, supposing them to arise, certain general consequences must follow. Neither in the tables which I have constructed must absolute accuracy be expected; the decimal mode of calculation which I have adopted, did not admit of precision, without a degree of labour, which would have been disproportionate to the object which I have in view. The different series of densities are at all times in geometrical progression to the heights, and they should be precisely compounded of the pressures and temperatures with which they are joined, reckoning as 1 the specific gravity of air at 32°, and under a pressure of 30 indies of Mercury of the same temperature. These conditions the tables generally fulfil to one or two places of decimals; but as the calculations have been made one under the other, the remainders, which were of no consequence in the original sums, have become appreciable quantities by the various successive multiplications and divisions. The errors, however, are in no case so large as to interfere with the general principles which it is my aim to establish.

We have hitherto contemplated these changes with reference to the particular column of the atmosphere in which they had their origin; we must now endeavour to trace their effects upon those

[page] 38

with which they are connected. We must recollect, that it has been established, as a principle, that the equal height of the Barometer, in every situation upon the surface of the sphere, was dependant upon the maintenance of the equatorial and polar currents, with a certain determinate velocity in the different parts of their courses, and that no disproportionate alteration or interruption in these could take place without a corresponding effect upon the mercurial column. Now, upon a reference to Tables VII. and VIII., it will be found, that, to keep the Barometer at 30.000 inches under Latitude 40, a current is required of the force of 0.854 inches towards Latitude 30, counterbalanced by one in the contrary direction, of the force of 0.291 inches at the elevation of 30000 feet: but by the unequal alteration of temperature shewn in Table XIV., the current at the surface is increased to 2.07 indies, and continues with diminishing force to the height of 30000 feet in the same direction. It is clear, therefore, that a much greater drain takes place upon this Latitude, without an adequate compensating supply,—the Barometer must, therefore, fall throughout the column. This fall, it will be observed, may take place without any disturbance of the temperature. The atmosphere incumbent upon Latitude 20, will be similarly affected by the same change of temperature at Latitude 30. In its original state, the lower polar current flows upon the surface with a force of 0.648 inches, and feeds this column with a supply of air. It is balanced at the height of 30000 feet by an equatorial current of 0.170 inches.

[page] 39

The course of the former is now reversed, and the drain is increased in the contrary direction. A rapid fall of the Barometer must, therefore, ensue.

On the other hand, an increased afflux of air, beyond the usual supply, to any portion of the atmosphere, occasioned by the expansion of any of the neighbouring parts, must cause an increase of density; and the equiponderant column will, of course, be lengthened. It is easy to perceive, that these secondary effects must widely extend the influence of the original disturbing cause; and it is obvious, that every depression of the Barometer must be accompanied by an equivalent rise in distant parts of the elastic medium, and vice versâ. The local impulse extends its influence in this, as in all other fluids, by the laws of undulation. The mean pressure, at any moment of time, of all the waves upon the surface of the sphere, will be the pressure of the atmosphere at rest, and the average of a large number of oscillations at any particular spot will approximate to the same quantity.

I have thus attempted to shew that the proximate cause of the fall of the Barometer, at the surface of the sphere, in an atmosphere constituted as our postulates have required, may be an increase of elasticity in its upper parts, beyond what is due to their respective elevations; and of its rise an analogous decrease in the same situations. These changes I have endeavoured to exhibit, as affecting directly the columns themselves in which the temperature varies, and remotely the adjoining columns, from their influence upon the lateral currents. This influence we have hitherto contemplated as extend-

[page] 40

ing only in the direction from the poles to the equator or from the equator to the poles; as if the changes of temperature which we have supposed, had extended, under the same parallel of latitude, round the sphere. The course of our argument now requires that we should shortly consider the same changes, as bounded in their longitudinal as well as in their latitudinal extent For this purpose, we must sup-pose our sphere to be divided into sections of 10 degrees at right angles to the former division.

Now, the arrangement, represented in Tables VII. and VIII., resulted from the temperature of the sphere itself, which we supposed to increase in heat in a regular progression from the poles to the equator; and the set of the aërial currents, under such circumstances, must necessarily be from north to south, and from south to north. But let us imagine that the local increase of heat, which is represented in Tables IX. X., is not only confined to 10 degrees of latitude, but also to 10 degrees of longitude: it will then be obvious that there will be currents established at right angles to the former winds; and they will all tend to compensate the irregularity which has been introduced. The following Tables present the results of the calculation of these eastern and western currents.

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TABLE XIX. Shewing the Effects upon the Atmospheric Columns of a general Alteration of Temperature in the direction of the Longitude.

Longitude 20 and 360 Longitude 10.
Height. Elasticity. Sp. Gravtiy. Temp. Elastictiy. Sp. Gravity. Temp.
0 30.000 1.00000 32. 30.000 .93960 60.8
5000 23.949 .82656 14.8 24.215 .78533 44.6
10000 19.106 .688321 -3.1 1.531 .65639 27.9
15000 15.229 .56472 -22.4 15.739 .54863 10.
20000 12.044 .46677 -43.6 12.673 .45856 -9.4
25000 9.579 .38582 -67.5 10.162 .38327 -31.2
30000 7.566 .31890 -95.1 8.135 .32035 -55.9

TABLE XX. Shewing the Force of the Currents occasioned by the preceding Alterations.

Longitudes 360 or 20 and 10.
Height. Elasticity. Sp. Gravity. Balance.
0 0 +1.929 +1.929
5000 -.266 +1.301 +1.035
10000 -.425 +0.337 +0.412
15000 -.510 +0.496 -0.014
20000 -.629 +0.251 -.378
25000 -.583 +0.076 -0.507
30000 -.569 -0.043 -0.612

But that portion of the atmosphere which thus rushes to supply the place of the air which has been rarefied within the prescribed limits, is already, as

[page] 42

we have seen, in motion from the pole to the equator: its course will, therefore, be intermediate between the two forces which impel it, and it will reach its destination with a northern or southern deflection. It will not be necessary to enter into the calculation of other cases of disturbance with the same limitations of longitude, as it is obvious that analogous effects must follow; and easterly or westerly currents, differently modified in direction and force, must result, whenever partial alterations of density take place, either from the immediate effects of expansion, or from change of mechanical pressure. The examples which have been adduced will be sufficient to illustrate the nature and operation of certain general principles, whose application is almost infinite.

There is, however, one more view of the subject which may assist us in our after-application of these particulars to the intricacies of atmospheric changes. Referring back again to Tables VII. and VIII., let us now suppose an increase of ten degrees of temperature to take place along the whole extent of any given meridian, and a decrease of equal amount at the opposite point; and let all the meridians on each side be similarly affected in a ratio between the two. The distribution of heat upon these two lines, and the two intermediate, would be as follows, for every ten degrees of latitude.

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TABLE XXI. Shewing the Distribution of Heat all over the Sphere, upon the supposition of a gradual Increase of Heat between the opposite Meridians.

Longitude
270
Longitude
0
Longitude
90
Longitude
180
Lat. 90 0 10. 0 -10.
Lat. 80 3.2 13.2 3.2 -6.8
Lat. 70 9.6 19.6 9.6 -0 4
Lat. 60 19.2 29.2 19 2 +9.2
Lat. 50 32. 42. 32. 22.
Lat. 40 48. 58. 48. 38.
Lat. 30 60.8 70.8 60.8 50.8
Lat. 20 70.4 86.8 70.4 60.4
Lat. 10 76.8 86.8 76.8 66.8
Lat. 0 80. 90. 80. 70.

This increase of heat, we are further to imagine to take place, throughout the respective columns, in so gradual a manner as not to affect the barometer at their bases.

Then will there be two currents established upon the surface of the sphere, in opposite directions on either side of the cold meridian towards the hotter, with a force of 1.304 inches; or rather, the body of the air, which was before in motion from north to south, will now be deflected with this force to the east and west; and the whole lower atmosphere, excepting upon these lines where the effect would be null, will move from the poles to the equator,

[page] 44

with a greater or less bend to the east and west. If the cause producing this variation of heat be supposed to move round the sphere from east to west, then will every meridian, in succession, be subjected to alternations of eastern and western currents.

All our reasoning has hitherto been applied to a sphere at perfect rest in itself: we will now give it motion, and suppose it to turn upon its poles with a certain regular velocity from west to east We will, however, continue the supposition of equal gravity, and put out of consideration, for the present, the effect of centrifugal force. Since this rotatory motion must be greatest at the equator, and is directed eastward, the air, in its passage from the poles, not having attained the maximum velocity, will have a relative motion westwards; and hence, the combined motion of the wind will be directed in the northern hemisphere from north-east to southwest; and in the southern, from south-east to northwest. Whenever this apparent tendency coincides with an actual impulse in the same direction, derived from other sources, it will augment its force; and when opposed to one in a contrary direction, it will tend to neutralize it. Thus, in the supposition which has been made above, of an accession of temperature upon the whole of any one meridian, the current, which we found would thence arise from the east towards that meridian, would be increased by this further mechanical impulse; while the western current on the opposite side would be decreased, if not annihilated.

But as the lower polar current would thus have a

[page] 45

relative western direction, with regard to the motion of the sphere itself, so the upper equatorial current would have an absolute movement in the contrary direction. The particles of air, which are transported from the polar regions to the equator, have not time to assume the velocity of the different parallels of latitude as they reach them; and are, therefore, necessarily behind them, as they revolve. To other bodies, therefore, possessing that velocity, they oppose a resistance which appears to come from the eastern quarter. Those, however, which are transported above from the equator to the poles, have an excess of absolute motion from west to east above those parts of the globe towards which they are carried. Now, as the heating power, which is the main-spring of all the motions of the atmosphere, is supposed to be in the sphere itself, it follows that the upper parts, which are most remote from it, will become cooled; while those which are nearer to, or in contact with it, maintain their proper temperature. As they cool, they of course become specifically heavier and descend; their place being supplied by the subjacent warmer strata. Another kind of circulation is thus established in a direction perpendicular to the horizontal currents which we have been considering, and in no wise interfering with them. If we consider, therefore, the motion of any one particle of air, we shall find that its course has an angular direction compounded of these two motions.

[page] 46

Fig. IV.

Let A B, Fig. IV., represent an upper, and C D an under, current, moving with equal velocities in opposite directions. Let us take any particle in the higher E, and suppose it to assume a density greater than is due to its situation, then will its course be from E to F, in an inclined direction, resulting from the two forces which solicit it If we follow it further, we shall find that one of these forces ceases its action, and gives place to another in a contrary direction; its path will then be from F to G, resulting from the other force, which continues its action, and the new impulse which it now receives. In this manner may an interchange of particles be kept up between the two principal currents, without at all interfering with their courses. We have, however, just observed, that the upper equatorial current is endued with a movement of rotation from west to east greater than that of the polar latitudes towards which it is carried: this impulse, being unopposed, must be borne by the particles in their descent from the higher to the lower stream; and the consequence must be, that the latter will be deflected from its course; and the northern current will receive a westerly direction at the point where this influence reaches its stream with sufficient power.

[page] 47

We have thus, by gradual stages, obtained some insight into the properties of an atmosphere of permanently-elastic fluid, surrounding and gravitating towards a sphere of unequal temperature, increasing from the poles to the equator, and revolving upon its axis with equal and definite velocity. Its state of equilibrium, which it must always be striving to attain, by whatever obstacles opposed, is maintained by two grand systems of currents, equally balanced, varying in force and direction, and onginating partly from differences of density, and partly from relations to the rotatory movement The principal circulations are in a horizontal direction from the poles to the equator, in the lower system; and from the equator to the poles, in the upper: and in a vertical direction, a constant interchange of particles between the superior and inferior strata. These motions are effected by means of differences of temperature and consequent differences of density. Subordinate to them, are two partial and local currents, which, as they arise from the rotation of the axis of the sphere, are in a direction at right angles to the former, and opposed to each other. The conjunction of these forces produce certain deflections from the primary directions; but, as the upper and lower systems are oppositely affected throughout, they compensate each other's motions, and their combined pressure is the same in every part. In this nicely-balanced order of things, we have seen how slight irregularities of temperature might produce great disturbances, and we have traced various expansions and contractions, which acting unequally upon the antagonist currents, would destroy

[page] 48

the adjustment of their several velocities. Accumulations in some parts, and corresponding deficiencies in others, would hence arise, the amount of which would be weighed by the barometer. These, in seeking to regain their proper level, and struggling to restore the equilibrium, would give rise to temporary and variable winds, which would modify the regular currents, and often reverse their courses.

Having established these several particulars, as I think, upon fixed and acknowledged principles, I shall now proceed to investigate the second division of my proposed inquiry.

[page] 49

PART II.

On the Habitudes of an Atmosphere of Pure Aqueous Vapour.

IN the first part of this treatise I have considered the habitudes of an atmosphere of perfectly dry and permanently-elastic fluid: the second branch of the investigation leads me to the consideration of an atmosphere of pure unmixed aqueous vapour. I shall first contemplate it, in this as in the former case, as surrounding a sphere of uniform temperature throughout, which we must now suppose to be covered with water. This temperature we will fix at 32°, as in our first hypothesis of the permanently-elastic fluid. It will not be necessary here to make any distinction between water in its fluid, and in its solid, state: ice, as well as water, it has been well ascertained, yields vapour of elasticity proportionate to its temperature; the general term may, therefore, be employed without impropriety.

Now the elastic force of steam, for the different degrees of heat within the range of atmospheric temperature, has been determined with very great precision. The following Table, extracted from the works of Mr. Dalton, includes the results as far as they are necessary to our present purpose:—

E

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TABLE XXII. Shewing the elastic Force of Vapour for every Degree from 0° to 90°.

Temp. Pressure. Temp. Pressure. Temp. Pressure. Temp. Pressure.
0 0.064 22 0.139 45 0.316 68 0.676
1 .066 23 .144 46 .328 69 .698
2 .068 24 .150 47 .339 70 .721
3 .071 25 .156 48 .351 71 .745
4 074 26 .162 49 .363 72 .770
5 .076 27 .168 50 .375 73 .796
6 .079 28 .178 51 .388 74 .823
7 .082 29 .180 52 .401 75 .851
8 .085 30 .186 53 .415 76 .880
9 .087 31 .193 54 .429 77 .910
10 .090 32 .200 55 .443 78 .940
11 .093 33 .207 56 .458 79 .971
12 .096 34 .214 57 .474 80 1.000
13 .100 35 .221 58 .490 81 1.040
14 .104 36 .229 59 .507 82 1.070
15 .108 37 .237 60 .524 83 1.100
16 112 38 .245 61 .542 84 1.140
17 .116 39 .254 62 .560 85 1.170
18 .120 40 .263 63 .578 86 1.210
19 .124 41 .273 64 .597 87 1.240
20 .129 40 .283 65 .616 88 1.280
21 .134 43 .294 66 .635 89 1.329
44 .305 67 .655 90 1.360

[page] 51

According to this Table, with the temperature at 32°, the equiponderant column of mercury would be .200 inch; and it would be the same at every part of the surface. The density of the vapour, like that of the gaseous atmosphere, and for the same reasons, must decrease in a geometrical progression for equal perpendicular distances, and the temperature will decline with it. The ratio, however, of its diminution will be very different.

To exemplify the calculation, we will take the height of 10000 feet. We must first find the height of an homogeneous atmosphere of such vapour equivalent to .200 inch of Mercury. Its specific gravity, compared to dry air, is as 2.317 to 557.800, or as the weights of a cubic foot of each respectively; therefore, 2.317: 557.800:: 10500: 2527794 then 2527794 × 0.2 inch=505558 inches=42l29 feet,

Height of Hemogeneous Vap. Given height. Modulus of Logailthm. Diff. of Log. of Denaltles.
and 42129: 10000:: .4342954: .1030868
Density. at 320
Log. of .200 =.3010300
-.1030868 Density, at 250
=.1979432 =Log. of. 157

At the height, therefore, of 10000 feet, the mercurial column, which the atmosphere would support, would only be .157 inch, and the constituent temperature of vapour of this degree of elasticity is 25°. In this manner the following Table was constructed of the elasticity, density, and temperature of such an atmosphere as we are now contemplating, at different heights:—

E 2

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TABLE XXIII. Shewing the Decrease of Density and Temperature in an Atmosphere of Aqueous Vapour, of the force of .200 inch at different Elevations.

Height in Feet Elasticity. Density. Temp.
0 0.200 1.000 32
5000 .177 .890 28.5
10000 .157 .790 25
15000 .140 .708 22
20000 .124 .636 19
25000 .110 .577 16
30000 .100 .518 13

With such an arrangement, there would be perfect equilibrium, and consequently perfect rest, all, over the sphere. No precipitation or evaporation would take place, and the atmosphere would remain transparent and undisturbed. Such also must be the state to which an atmosphere of vapour would strive to attain, notwithstanding any obstacles which might be opposed to it. Hence we may also infer that, if condensation were to take place in any part of such an atmosphere, evaporation must follow in other parts to maintain the balance of forces; and conversely, that evaporation must be accompanied by precipitation.

Should the temperature of the sphere rise gradually and equally over all its surface, the elasticity of the steam would increase with it, without disturbance; and, following its own law of decrease for its different elevations, would remain perfectly transparent.

[page] 53

In considering the second modification of circumstances, that, namely, of the temperature of the sphere increasing from the poles to the equator, we must first observe, that a pure unmixed atmosphere of vapour could not follow such a gradation. The elasticity of the whole would be determined by that of the lowest point; and the water would distil from the hottest point to the coldest, with such rapidity, as to occasion strong ebullition at the former. The condensation of vapour may be effected, not only by decrease of temperature, but by increase of pressure: it is not necessary, therefore, that it should pass from the hottest to the coldest point to be precipitated, which would be a gradual process, but the elastic force, arising from an increase of density at one extremity, would instantly be felt at the other; the impression being conveyed as through a spring. The best illustration of this effect may be derived from the Cryophorous of Dr. Wollaston, in which the force of the vapour is so much reduced by the cold applied to one extremity of the instrument, as speedily to produce congelation at the other by the rapidity of the consequent evaporation. For our present purpose, however, we must put out of our consideration the rapidity of this action, and imagine the passage of the vapour from one point to another to be so mechanically retarded, as to enable it to assume the gradations due to the heat of the sphere. We may then estimate the relative force and pressure of two of the perpendicular columns at different stations. The following table represents the state of the vapour at the equator supposing the temperature 80° as before.

[page] 54

TABLE XXIV. Shewing the Decrease of Density and Temperature in an Atmosphere of Aqueous Vapour of the force of 1.000 Inch at different Elevations.

Height. Elasticity. Density. Temp.
0 1.000 4.571 80.
5000 .897 4.115 76.5
10000 .804 3.682 73.
15000 .722 3.356 70.
20000 .648 3.026 67.
25000 .681 2.723 63.
30000 .521 2.487 60.

By comparing these results with the last Table, it will be observed, that, unlike the case of the permanently-elastic fluid, both the density and elasticity increase greatly with the temperature; and the consequence must be that the equatorial columns must press upon the polar throughout their length. A circulation will hence arise very different from that of the aërial currents. The vapour will flow in a mass from the equator to the poles, and, being necessarily condensed in its course, will return from the poles to the equator in the form of water. Great evaporation will constantly be going on at the latter station, and condensation at every other: so that the atmosphere, excepting at the equator, would be rendered turbid by perpetual clouds and rain. As in the case of the permanently-elastic fluid, the temperature of the sphere would, by this process, soon become equalized, did not our hypothesis provide

[page] 55

for its permanency: the equatorial parts would be quickly cooled by the evaporation, and the polar warmed by the heat evolved during the condensation.

It is further worthy of attention, that, the elasticity of vapour increasing nearly in a geometrical proportion for equal increments of heat, the decrease of temperature in ascending in this atmosphere will be in arithmetical proportion only. The diminution is very nearly three degrees for every 5000 feet.

Upon the hypothesis of the gradation of temperature before assumed, in the case of the gaseous atmosphere, the following Table will represent the corresponding elasticity and density of the vapour, at the surface of the sphere, for every ten degrees of latitude.

TABLE XXV. Shewing the Force and Density of an Atmosphere of Aqueous Vapour, for every Ten Degrees of Latitude, surrounding a Sphere unequally heated.

Poles. Latitude 80 Latitude 70 Latitude 60 Latitude 50
Elas. Density Temp Elas. Density Temp Elas. Density Temp Elas. Density Temp Elas. Density Temp
.064 0.340 0 .072 0.380 3.2 .089 0.466 9.6 .125 0.641 19.2 .200 1.000 32
Latitude 40 Latitude 30 Latitude 20 Latitude 90 Equator.
Elas. Density Temp Elas. Density Temp Elas. Density Temp Elas. Density Temp Elas. Density Temp
.351 1.700 48 .539 2.547 60.8 .731 3.403 70.4 .900 4.143 76.8 1.000 4.571 80

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Under these circumstances, the equatorial regions will remain perfectly transparent, white rain will continue to fall in every other situation, in proportion to the densities of the respective places and the decrease of temperature; and the supply of vapour will beentirely kept up by the evaporation at the equator. This circulation may be regarded as a species of regulated distillation. The height of the barometer decreases rapidly towards the poles from 1 inch to .064 inch, and the quantity of condensation is definite for each latitude; for the resistance to the passage of the vapour is supposed to be constant and equal.

Let us now imagine that the temperature of any particular latitude is raised to the level of that which adjoins: then will condensation cease at that particular place, evaporation will commence, and the atmosphere will become transparent. The quantity of water precipitated will be proportionally increased on the other side. If, on the contrary, the temperature be lowered to the standard of the latitude next above, the precipitation will be increased, and the higher latitude will be cleared. The following Table represents Latitude 30 under these two conditions.

TABLE XXVI. Shewing the State of the Atmospheres arising from Alterations of Temperature in any intermediate Columns.

Latitude 40
Cloudy
Latitude 30
Cloudy
Latitude 20
Cloudy
.351 1.700 48 .731 3.403 70.4 .731 3.403 70.4
Clear Cloudy Cloudy
.351 1.700 .351 1.700 .731 3.403 70.4

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Again—if the mechanical retardation of the flowing vapour, which we have imagined, were subject to variation, the quantity of evaporation and precipitation would be proportionate to the velocity of its passage: thus, supposing the evaporation from a given surface at a given temperature, and under a certain resistance, to be three grains per minute, it would be increased to six grains with half the resistance. It would be easy to apply these consequences to analogous cases, but it will not be required to trace them more particularly. The changes at the surface affect the whole of the superincumbent column equally, and the temperature of the vapour follows its own law of decrease. But what will be the consequence, if the vapour should be forced to adapt itself to a progression of temperature different from that of its own; and if from some cause or other (no matter at present whence originating) the heat of the upper regions should diminish at a greater rate than is due to the natural gradation?

Let us, for instance, suppose that the heat of the water upon the sphere is 80°, but that, at the height of 5000 feet above the surface, a temperature exists of 64°.4, which from that point follows the former decreasing scale. The water will have a tendency to throw off vapour of the same constituent heat as its own temperature; but the pressure above, being rendered too little by the influence of the forced degree of cold, to preserve the necessary elasticity below, the atmosphere will only possess the tension due to the lower degree; that is to say, the constituent temperature of the vapour will be only

[page] 58

67°.9. Evaporation must therefore ensue below, and its concomitant precipitation will take place above. The calculation of these effects has furnished the following tabular representation of their connexion:

TABLE XXVII. Shewing the Effect upon the Atmosphere of Vapour of a forced gradation of Temperature.

Height. Elasticity. Constituent Temp. of Vapour. Sensible Temp. State of Atmosphere.
0 .673 67.9 80 Clear
5000 .606 64.4 64.4 Cloudy
10000 .542 61 61 Clear
15000 .490 58 58 "
20000 .443 55 55 "
25000 .401 52 52 "
30000 .363 49 49 "

The consequence of this situation of things will be, that a cloud will be formed at the height which has been named: for the atmosphere will be forced upwards by the nascent vapour below, and will be condensed at this point. The cloud, however, supposing the process to be sufficiently gradual, would not extend very far downwards, for the water, during its precipitation, would be redissolved by the excess of heat in the lower regions, so that they might remain transparent and undisturbed. The ultimate effect would be that the temperature would be slowly equalized, and the balance of force restored. The

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water, in its circulation backwards and forwards, would act as a carrier of the heat, which it would abstract from the lower parts by its evaporation, and give out to the upper by its condensation. The atmosphere would thus gradually recover its state of equilibrium and repose. The upper regions, upon this supposition, remain clear, for there the regular gradation is undisturbed.

This part of the subject may, perhaps, derive some illustration from the following analogy and figure.

Let a and b, Fig. 5, represent two glass globes connected together by the tube c:—d is a mercurial gauge for the purpose of measuring the elasticity of any included vapour. Let us suppose the apparatus to be free from air, and water to be included in a; and let the temperature of both the globes be 80°. The column of mercury supported in d will then be equal to 1 inch, and no evaporation or condensation can take place. Let us now imagine the globe b to be suddenly cooled down to 32°. The mercury in the barometer d will instantly fell to .200 inch; the water will rapidly evaporate, and be condensed in b, and if both the temperatures be maintained, will entirely pass from a to b. The difference,

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between the force of vapour at 80° and 32°, will thus become the measure of the force of evaporation.

Now let Fig. 6 represent four globes, connected together in the same way as the two in Fig. 5. The whole apparatus is supposed to be free from air, and the ball a to contain water. The temperature of each is to be maintained respectively at 80°, 64°, 48°, and 32°. The gauges would then all denote a tension of .200 inch, and the water would distil over rapidly into d. The vapour, in its passage, meets with no obstruction; and the effect is necessarily the same as in the last case, where no intermediate receivers were placed. But, should the vapour be retarded in its course by any obstacle of a mechanical nature, as if the connecting tubes e f and g were packed with sand or cotton, the result would be very different. At the commencement of the process the rising steam would assume the elasticity necessary to enable it to pass the intervening obstructions into the globe d, and, whatever this elasticity might be, when in d, it would be reduced to the force of .200 inch. With this force it would, therefore, press upon the surface of the water in a. The nascent steam has now, not only the me-

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chanical impediment to overcome, but this additional pressure; so that its elasticity, and consequently its temperature, must rise in proportion. In c, however, it necessarily assumes the temperature of that vessel. A greater degree of elasticity, from c to a, now presses upon the fluid, and the force of the generated steam must again rise. It is again partly condensed in b, whose temperature will not support the higher degree, and the increased tension is exerted from b to a. The different gauges, b c and d, will thus denote the respective elasticities .597 inch, .351 inch, and .200 inch, appropriate to their several temperatures. The differences of force will denote the rate of evaporation in each.

Such mechanical obstruction, as we have supposed, would oppose itself to the free passage of vapour in motion, but would exert no pressure or influence upon it in a state of rest; and if we were to imagine that all the water had distilled over from a, and were included in d, the gauges would all stand permanently at .200 inch.

Let us now apply this to our atmosphere.—We have already proposed the simple case of a sudden decrease of heat at one stage of its height, by which condensation was produced; the elasticity was thereby reduced to the degree appropriate to that temperature at that elevation, and evaporation commenced from the surface. This evaporation was proportionate to the difference between the elasticity of vapour of the temperature of the sphere, and the elasticity of the superincumbent mass. We will now suppose that the rapid decrease of temperature continues throughout the column, and that

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at the following stages of its height it is forced to adapt itself to the annexed progression.

TABLE XXVIII.

Height. Temperature.
0 80.
5000 64.4
10000 48.4
15000 31.4
20000 12.8
25000 – 7.6
30000 –30.7

The elasticity could not then exceed .043 inch upon the surface: the evaporation would consequently be excessive, and its force would almost amount to explosive violence; while the condensation above would be proportionate, and the precipitation would resemble a water-spout in its effects. We must now provide (and by what means we will not now stop to inquire) some obstacle by which the course of the vapour may be retarded in its ascent, in a similar way to that which we have imagined in the glass globes,—then may the condensation take place gradually and at different heights. The relative distances of these points of precipitation will depend upon the force of the vapour, and the greater or less facility with which it overcomes the mechanical obstruction. For the scale of temperature laid down, the following table would represent an adequate balance of evaporation and condensation, with the appropriate degrees of elasticity between the points.

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TABLE XXIX. Shewing the Effects of a further forced Progression of Temperature upon the Atmosphere of Vapour.

Height. Sensible Temperature. Constituent Temp. of Vap. Elasticity. State of Atmosphere. Force of Eraporatton.
0 80 67. 9 .673 Clear 327
5000 64. 4 64. 4 ×.606 Cloudy 467
10000 48. 4 19. 124 Clear
15000 31. 4 16. .112 Clear
20000 12.8 12.8 ×.100 Cloudy 80
25000 –7.6 –20.7 .027 Clear
30000 –30.7 –30.7 ×.02 0 Hazy

The last division of this Table gives the relative force of evaporation at the different points, supposing the total effect, if unopposed and sudden, to be 1000, and the same numbers will represent the comparative amount of precipitation, at the several intervals of condensation, or the relative densities of the three clouds. In this manner the struggle between the elasticity of the stream, and the condensing power of the cold, is divided and moderated; and the whole process becomes so gentle as quietly to restore the balance of force and temperature, provided the counteracting cause be not of a permanent nature. The moisture, falling gradually back into the excess of heat below, is converted into vapour of higher force, which pressing more upon the inferior strata, proportionably raises their densities.

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From these considerations, it would appear, that, in any single column considered by itself, clouds of greater or less densities, and evaporation of greater or less force, must be the consequence of a temperature decreasing in a more rapid progression than is due to the law of aqueous vapour.

While the atmosphere is in the state represented in Table XXIX., let us now contemplate the effects of a general reduction of sensible temperature upon the constituent temperature, and the different points of precipitation. We will suppose the fall to take place gradually, and to amount to 10 degrees. In the first place, the elastic force upon the surface will not be diminished, but will approach the point of precipitation within three degrees. A plane of condensation will be established between the surface and the height of 5000 feet. So, likewise, the vapour from 9000 to 14,000 feet will not be disturbed, but the second plane of condensation will descend from 18,500 feet, to an intermediate position between that elevation and 14,000 feet. The shifting of these planes would not be sensible at the surface; for the light precipitations, which would accompany their slow subsidence, would be expended in equalizing the temperature.

We must next contemplate these various phenomena, hitherto considered as confined to a single column, in connexion with adjacent sections. Let us take, as an illustration, the equatorial column of 80° temperature, in the state in which we have just considered it, and the adjoining one of 76°. 8. The flow of the lateral currents may be determined by the following Table:—

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TABLE XXX. Shewing the Stale of the Atmosphere occasioned by the Intermixture of Lateral Current.

Latiude 10 Latiude 0
Height. Senaible Temperature. Constituent Temperature. Elasticity. State of Atmosphere. Sensible Temperature. Constituent Temperature. Elasticity. State of Atmosphere.
0 76. 8 51 .388 Clear 80 67. 9 .673 Clear
5000 61. 1 48 .351 Clear 64. 4 64. 4 .606 Clear
10000 44. 9 45 .316 Cloudy 48. 4 19 .124 Clrar
15000 27. 7 12 .096 Clear 31. 4 16 .112 Clear
20000 9. 3 9. 3 .087 Cloudy 12.8 12.8 100 Clear
25000 -11. 6 -3 2 .019 Hazy - 7.6 -27 0.27 Clear
30000 -35 . -3 5 .016 Hazy -30. 7 – 30. 7 .020 Clear

F

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In this Table, the first point of condensation above, in the equatorial division, is supposed to take place at the height of 5000 feet; while, at latitude 10, it is fixed at 10,000 feet; and it will be seen, that up to the former elevation, the vapour of the first column is of much greater elasticity and density than that of the latter: it will, consequently, flow towards it with considerable force. No cloud will be formed, as before at the point of condensation, for the supply arising from the evaporation at the surface, will be carried off in a lateral direction; or, if previously formed, would soon be dissipated by the same action. Nor would the transparency of latitude 10 be affected up to this height; for the current which it would receive, would, in constituent temperature, still be below what its sensible heat would maintain. But above this line a dense cloud would be precipitated. A counter-flow of small extent towards the equator will be established at 10,000 feet; and above this, again, the pressure will return to the first direction. The constituent temperature of the returning current, being below the temperature of the elevation, the transparency of the equatorial column will be preserved throughout. These lateral currents are supposed to take place under the same mechanical retardation as the ascending vapour.

It would be easy to multiply and vary these illustrations; but enough has been done, to shew generally that the necessary condition of transparency, in any vertical section of an atmosphere of pure vapour, in which, from some extraneous cause,

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the temperature diminishes faster than the natural progression, is, that the quantity generated from the evaporation, necessarily accompanying such circumstances, should be carried off to adjoining regions.

Our hypotheses have hitherto been framed upon the assumption, that the sphere round which the aqueous atmosphere has been diffused, was covered with water, whence a continual supply of vapour would flow equivalent to every increase of temperature. Let us now suppose that water is only partially diffused, and that the uncovered portions are absolutely dry. Vapour, out of the contact of water, is affected in the same way as the permanently elastic fluids, by variations of temperature; that is to say, it contracts or expands 1/480th part of its volume for each degree of change, above its point of precipitaion, by Fahrenheit's scale. If a current, therefore, were to pass over a dry space, heated to a degree higher than itself, the same changes in its constitution would take place, in miniature, as we have already traced in the dry atmosphere. Its density would diminish, While its elasticity would retain the same upon the surface, and be increased at higher stations. The following Table presents us with the different degrees of elasticity in a column of the constituent temperature of 80° heated to 90°.

F 2

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TABLE XXXI. Shewing the Effects of Expansion upon Vapour heated beyond its Dew-point.

Height. Elasticity. Constituent Temperature. Sentible Temperature.
0 1.000 80 90
5000 .899 76.5 86.5
10000 .808 73 83
15000 .727 70 80
20000 .653 67 77
25000 .587 63 73
30000 .528 60 70

Such modifications would necessarily ensue in the cases which have already been considered, of the constituent temperature falling below the sensible heat; but, by comparing this Table with Table XXIV., it will be seen, that the total effect at the greatest extreme does not exceed .007 inches; and it will be unnecessary, in the general view which we are now taking of the subject, to introduce a correction to such a small amount.

In the case of vapour becoming heated in this manner, out of the contact of water, it may reach its point of deposition at a high elevation without producing any sensible cloud; for, although it would be slowly precipitated, it would be instantly restored to the elastic form by the excess of heat in the inferior strata; and no accumulation could be formed for want of supply from the dry surface below. A slight haziness might possibly be the result.

Let us now imagine, a stream of vapour, of known

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density, filtering its way laterally through the resisting obstacle which we have supposed, from one part of the sphere, which is covered with water of a certain temperature, to another which is perfectly dry, and of equal or superior temperature. As it arrives at the latter point, it will diffuse itself rapidly over the dry space; and its elasticity, being no longer confined by an incumbent atmosphere of like density, will be reduced, and it will assume that force which its own diffusion will enable it to maintain. Or a stream of vapour, of high elasticity, flowing into a space where there already exists an atmosphere of inferior force, will be reduced in density to that of the general mean. In Table XXXII. are represented two contiguous columns of vapour; the first incumbent upon water of the temperature of 70°, and the second upon dry land of the temperature of 80°. It is obvious that the former will flow into the latter, but will no longer be distinguishable by its constituent temperature of 60°, but will be reduced to the standard of the second column 32°. The elasticity, however, of this column will rise with the diffusion of the first.

TABLE XXXII.—Shewing the Effects of the Diffusion of Vapour of High Elasticity from Water, over a dry Surface.

Water. Dry Earth.
General Temp. Const Temp. Elasticity. General Temp. Const Temp. Elasticity.
70 60 .524 80 32 .200

But the surface upon which an atmosphere of any particular density rests, may be neither water, nor yet perfectly free from it, but we will imagine

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it to be earth differently imbued with moisture and variously heated—then a partial supply, varying in quantity in different placed but of the same degree of density, would take place, and clouds of more or less opacity would be formed, at corresponding situations, in the planes of deposition above. The following Table will, tend to illustrate these positions.

TABLE XXXIII. Shewing that the Elasicity of Vapour, yielded by different Surfaces variously heated, is governed by the incumbent Atmosphere.

Water, Temperature 60.8 Moist Earth, Temperature 70 Dry Earth, Temperature 80
Height. General Temp. Constant Temp. State of Atmosphere. Force of Evap. Constant Temp. State of Atmosphere. Force of Evap. Constant Temp. State of Atmosphere. Force of Evap.
0 60.8 34 Clear .368 34 Clear 507 34 Clear 786
5000 44.6 31 Clear 31 Clear 31 Clear
10000 27.9 28 Cloudy Density .368 28 Cloudy Density .092 28 Hazy?
15000 10. -6.4 Clear -6.4 Clear -6.4 Clear
20000 -9.4 -9.4 Cloudy .126 -9.4 Hazy .021 -9.4 Clear
25000 -31.2 -31.2 Hazy .020 -31.2 Clear -31.2 Clear
30000 -55.9 -55.9 Clear -55.9 Clear -55.9 Clear

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The flame general temperature is here supposed to prevail momentarily in every part of an atmosphere of equal force; resting upon a surface covered with water in one part, with moist earth in another, and dry in a third, and varying moreover in heat in the three situations. The first point of precipitation at placed at 10000 feet. The water upon which the first part of the column rests, is of the same degree of heat, as the general temperature at the surface; the force of evaporation is 368, and as the supply is equal to the force, the density of the cloud is 368. The moist earth, upon which the second portion rests, is of the temperature of 70°, which makes the force of evaporation 507: but less steam being given off from the earth, than from the water, the quantity of the precipitation is proportionally diminished. It is calculated in the Table at one fourth. The dry surface, which supports the third portion, is heated to 80°, and yields no vapour: the evaporating force, which is equal to 786, is wholly unapplied, and no cloud can therefore be maintained. The higher points are subject to the same modifications. The temperature of the evaporating surface regulates the quantity of water raised in vapour, the tension of the preexisting atmosphere determines its elasticity.

I shall here conclude the separate consideration of the properties of aqueous vapour, applicable to my design. It is impossible not to perceive that I have already been obliged to anticipate some particulars, which would have fallen more consistently with my intended division under the third section: but I have restricted the inquiry as much as pos-

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sible to the single object in view. It will, however, have been anticipated, that the mechanical retardartion of the motions of the vaporous atmosphere, together with the forced progression of temperature, which have been so often referred to, belong to the state of mixture with the gaseous atmosphere; and I may now further connect the preceding remarks by observing, that, in the operations of the aqueous steam, a power is developed fully adequate to produce the disturbances of temperature hypothetically proposed in the examination of the permanently-elastic fluid.

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PART III.

On the Habitudes of an Atmosphere of permanently-elastic Fluid, mixed with aqueous Vapour.

HAVING separately considered some of the properties of simple atmospheres of permanently-elastic fluid and of aqueous vapour, most essential to the object of our inquiry, we may now proceed to investigate the compound qualities of a mixture of the two, and their mutual relations so combined.

The properties, which each possessed in its separate state, will be retained in this connexion unchanged, and the two fluids will exercise no further action upon each other, than a mechanical opposition when in motion. The particles of steam, in penetrating the interstices of the permanently-elastic fluid, experience the same species of retardation as exists in their flowing through the pores of sand or cotton. When a state of equilibrium is attained, this mutual action ceases, and the particles of each press only upon those of their own kind. There are, therefore, two principal points of view, under which such a mixture may be regarded,—one, in which the particles are in a state of equipoise amongst themselves; and the other, where they are

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seeking an equilibrium by means of intestine motion. With respect to the first, there is no distinction between such a complete mixture and that of two or more permanently-elastic fluids; and it may be regarded like a mixture of gases, as an homogeneous fluid.

We will now inquire what would be the natural state of such an atmosphere, surrounding a sphere of uniform temperature throughout?

Before we can answer this question satisfactorily, we must consider what are the effects of mixing known measures of the gases, with vapour of different degrees of force.—The first effect is increase of bulk, under equal pressure in the permanently-elastic fluid; not in proportion to the measure of vapour added to it, but in proportion to its elasticity. Thus, if we mix a cubic foot of dry air, of the temperature of 212° and of the elasticity of 30 inches, with as much steam as would rise in the space of a cubic foot at the same temperature, and consequently of the force of 30 inches, the mixture would occupy the space of two cubic feet for 30 inches: 60 inches:: 1:2.

So, if we mix a like measure of air, of the temperature of 32° and of the elasticity of 30 inches, with as much vapour as would form in the same space at the temperature of 32°, and, consequently, of the force of only 0.200 inch, the bulk of the gas will only be increased .00666 of a cubic foot.

For 30.000: 30.200:; 1.: 1.00666

The case is exactly analogous to what would happen from the mixture of gases, of different degrees of elasticity, A cubic foot of air, of the elasticity of 30 inches, added to another cubic foot of

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the same elasticity, will have its volume doubled; but a cubic foot of air, of the elasticity of only .200 inch, being added to a cubic foot, of the elasticity of 30 inches, will only increase its volume .00666 of a cubic foot.

The second result is, that the specific gravity of the gas is decreased; but not exactly in proportion to its expansion: for while the vapour dilates its parts, it adds its own weight to the mixture. But this weight, though increasing with the elasticity, being, in all cases, less than that of an equal bulk of common air, decrease of density must follow. The diminution becomes greater with every increment of temperature.

Let us imagine an homogeneous atmosphere of air, of the temperature of 77°, and 30 inches pressure: its specific gravity, compared to air at 32°, would be .00626:1.00000. Let us suppose this to be mixed with an atmosphere of vapour of the same temperar tane, and .910 inch force: the specific gravity of the mixture under equal pressure would be .89312: its total pressure would amount to 30.910 indies; and its height would be increased from 28775 feet to 30960 feet. But, as we have seen before, the mean temperature of an homogeneous atmosphere must fall, in assuming that gradation of density which is essential to its natural state; the quantity of vapour must, therefore, suffer a proportionate reduction. If we were to assume the scale of decrease, represented in Table VII., for a temperature at the surface of 77°, the corresponding diminution of its force would be as follows:—

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TABLE XXXIV. Shewing the small Quantity of Vapour, which could exist in an Atmosphere of Air, supposing it saturated throughout.

Height. Temp. Elasticity.
0 77 .910
5000 61 .542
10000 45 .316
15000 27.5 .171
20000 9.3 .088
25000 – 11 .042
30000 – 35 .016

The average quantity, therefore, of the vapour to this height, could not exceed 0.297 inch. We must further consider that the altitude to which we have hitherto followed our speculations, has comprised. but two-thirds of the total height of our atmosphere; the remaining third may, without any risk of error, be considered, from the lowness of its temperature, to be totally free from vapour. The mean pressure,, therefore, of steam would thus be reduced to .198 inch; and supposing such a state of circumstances to be possible, the barometer would only rise from 30 inches to 30.198 inches in the atmosphere surrounding a sphere of the temperature of 77°, by a change from absolute dryness to perfect moisture.

Nor would such a state of saturation constitute,. by any means, a natural condition of such an atmo-

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sphere as we are contemplating. Even if a general mixture could be effected in the proportions which we have above imagined, the elasticity of the steam at the bottom of the column would be greater than the weight which the upper strata would confine; so that being urged upwards it would be condensed by the temperature of the air, which decreases fester than the progression due to the vapour, and the barometer would not rise to the height just stated.

A state of complete mixture, in which all the particles would be at rest amongst themselves, cannot, therefore, exist in the natural atmosphere; and it can only, consequently, be regarded, in the second point of view distinguished above, namely, as in a state of intestine motion.

To place these particulars in a clearer light, let us trace the progress of vapour just beginning to form in a perfectly dry atmosphere. For this purpose we will imagine the temperature of the sphere to be 77°. The first arrangement will be such as is represented under latitude 10, in Table VII. Let us now imagine water suddenly to overflow the surface, and evaporation will instantly commence. No atmosphere of vapour exists to impede its progress, the nascent steam will, therefore, merely assume the degree of tension necessary to overcome the visinertice of the air which obstructs its motion. What this force may be, we have not, perhaps, sufficient data to determine. We must, for the present, fix it arbitrarily, and assume that, at the temperature of 77°, and pressure of 30 inches, it amounts to .200 inch. The constituent heat of vapour of this elasti-

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city is 32°, so that at the height of about 13,500 feet it would meet with ha point of condensation. An aqueous atmosphere of such degree of force being now established, fresh resistance to this amount is made to the progress of evaporation; and the elasticity of the rising steam must be doubled. Its constituent temperature is thus raised to 52°, and it cannot, therefore, pass the height of 7500 feet, without decomposition. The resistance upon the surface now amounts to .601 inch, to overcome which, vapour at 65° must be emitted. The first point of precipitation, in ascending from the surface, would thus be fixed at about 3600 feet. We may now further remark, that the diffusion of vapour does not cease at the height of 3500 feet, to which point we had first traced it; but the mechanical obstruction is proportionably reduced, and it is carried by successive stages to more lofty regions, where its tenuity is so much increased that it speedily eludes all observation.

With regard to the various points of condensation, it is probable, as was before remarked in the atmosphere of pure steam, that no cloud would be formed at any of them. The process of evaporation would be so gentle under these circumstances, that little above six grains of water would be raised per minute from the surface of a square foot; so that, as the gradual precipitation of this quantity took place between the different stages, it would instantly be re-dissolved by the excess of heat into which it would naturally incline to fell. The circulation thus becomes a process of equalization, by which the temperature of the up-

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par regions is raised: the heat which is abstracted below by evaporation is evolved by condensation, the pressure of the vapour is increased, and all the changes tend to that distribution of heat which we formerly contemplated as the natural state of an unmixed atmosphere of steam.

The average quantity of vapour, which would exist upon the hypothesis which we have just assumed, while the atmosphere maintained its proper progression of temperature, may be roughly approximated as follows:—A stratum, of the force of .616 inch, extends to the height of 3600 feet; another, of the force of .401 inch, reaches 3900 feet further; a third, of only .200 inch, stretches almost as far as both the former together; making a total of 13500 feet. The mean, therefore, to this point is nearly

.616/4 + .401/4 + .200/2 = .354 inch.

For the further distance of 17500 feet, we cannot greatly err in taking .064 inch, as the mean pressure, making the average to the height of 31000 feet, .209 inch. One-third of the atmosphere beyond this being considered free from vapour, reduces the mean to .139 inch.

The following diagram may possibly tend to elucidate the effects of an unequal addition of matter, or of unequal expansion in various parts of the same column of fluid.

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Fig. 7.

Let A B represent the column whose height and weight we will call 40. -Its four sections A B C D are each equal to 10. Let us suppose an addition of matter to take place in B, e f equal to 4; in D, g h equal to 3; in C, i k equal to 2; and in A, l m equal to 1. The total increase of weight and pressure at the bottom will be 10, or one-fourth of the original amount: the same as if the total amount had been equally distributed throughout the mass, or added at once to the top of the column, as n 0.

Again—if in the same column an unequal expansion were to take place in the four different sections, e f = 4, g h = 3, i k = 2, and l m = 1, the total increase of bulk n 0 would be the same as if the expansion had every where been equal, and the weight at the base remaining the same, that of the sections would be altered from 10. 20. 30. 40, to 8. 16. 24. 32 and 40.

In the case of the atmosphere, which we are considering, both these changes are combined: the barometer rises at the surface of the sphere, and the

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weight of the several strata is still further changed. The following Table exhibits the state of the barometer at equal altitudes, before and after the admission of vapour, in an atmosphere surrounding a sphere of the uniform heat of 77°; together with the temperature appropriate to the elevation and the dew-point.

TABLE XXXV.—Shewing the State of the Barometer, at equal Altitudes, in an Atmosphere of Air, before and after the Admission of Vapour.

Height. Temperature. Barometer. Atmos. without Vapour. Barometer. Atmos. without Vapour. Dew-Point.
0 76.8 30.000 30.139 65
5000 61.1 25.214 25.348 52
10000 44.9 21.193 21.318 32
20000 9.3 14.970 15.079 0
25000 -11.6 12.583 12.682 -35
30000 -35. 10.578 10.667 -35

Such, then, would be the new state of things, from the admission of water to the surface of the sphere: a state, however, which, notwithstanding the equality of the superficial heat, could not be one of permanent rest. A perpetual struggle would ensue between the temperature due to the density of the air, and the constituent temperature of the vapour, accompanied by perpetual evaporation below, and simultaneous condensation above. No winds or lateral currents would be established, but an increasing circulation in a vertical direction.

G

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By comparing this synoptic view with that presented by Table VII., it will be seen, that the specific gravity and elasticity of the air is but very slightly affected by this intermixture of aqueous vapour; so slightly, indeed, that the course and velocity of the currents, as represented in Table VIII. may be considered, without any chance of disturbing our main argument, as unaltered; and their balance to be that by which the barometer is maintained at an unvarying height. It will also be remarked, that, while the great aërial ocean is divided into two distinct strata, flowing in opposite directions from north to south and from south to north, the aqueous part, which is nearly confined to the lower current, presses in a contrary direction. The adjustment of these particulars remaining as now supposed, the compensating winds flow on, in the courses which have been described, and the balance remains undisturbed.

The admixture of vapour, which we have hitherto considered, has not yet affected the gradation of temperature, resulting from the decreasing density of the atmosphere in its upper parts; the process of evaporation, however, which has been described, must, in time, necessarily induce such an alteration. The stream, as it readies its point of condensation, must give out its latent heat, and during its precipitation, combining with a fresh proportion, it again ascends and again evolves it in the middle regions. It may thus be considered as carrying caloric from the surface of the sphere to higher strata; and it is obvious how a considerable section of any one column may thus have its temperature equalized and folly saturated with aqueous particles. The cur-

G 2

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rents thus become affected both by the expansive powers of the vapour and of the extricated heat—causes, the influence of which, so applied, must be partial, and cannot reach the higher regions. The unequal action must produce a fall in the barometer, as has been before explained.

As, on one hand, this effect upon the barometer is produced by the augmentation of the aqueous vapour; so, on the other, a rapid increase of the latter may be produced by a fall in the former. The mechanical resistance of the air must, of course, be increased by its motion in opposition. When this is stopped, as it soon is, by a trifling fall of the mercurial column, the vapour will rush forward with its whole force, retarded only by its filtration through the quiescent air; and the temperature of the higher latitude being unable to support its elasticity, precipitation must follow. From the operation of these causes, the temperature of the latitude is partially affected, the density of the air is still further reduced, and the aërial current is reversed. The course of the vapour is thus greatly accelerated, and abundant precipitation will follow.

The progress of the precipitated moisture, from the time when its first streaks would visibly shoot across the air, to the time when it would descend in rain upon the globe, is not without its interest. In proportion to the density of the vapour, no doubt, must be the magnitude of the condensed particles. When first formed in the higher elevations, the cloud would probably assume a light cirriform appearance; in lower regions the precipitation would be more dense, and the attraction of aggregation

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stronger; the mass would subside gently to a lower station, where the density of the air would oppose a greater resistance to its descent. Here, in a higher temperature, the cloud would again begin to be dissolved, and would assume a rounded and compact form; and thus the equalization of the temperature, and the diffusion of the vapour, would be carried on from several points at once. The different beds obey the impulse of the winds, and, as they sail along, enlarge the circumference of their action: till, at length, the natural equilibrium of the atmosphere can be no further curbed. The precipitations increase, the strata of the clouds inosculate, and the air no longer buoys up their load.

It will be convenient here to subjoin a synoptic view of the force of the aërial current, and the counter-pressure of the vapour in a mixed atmosphere, surrounding a sphere unequally heated in the manner already set forth.

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TABLE XXXVII. Shewing the Force of the different Currents in a mixed Atmosphere of Air and Vapour, between the Poles and the Equator.

Latitudes 90 & 80 Latitudes 80 & 70 Latitudes 70 & 60 Latitudes 50 & 40 Latitudes 60 & 50
Height. Wind. Vapour. Wind. Vapour. Wind. Vapour. Wind. Vapour. Wind. Vapour.
0 +.178 -.003 +.387 -.010 +.575 -.028 +.810 -.049 +1.034 -.103
5000 +.057 +.191 +.281 -.014 +.375 -.032 +.570 -.060
10000 +.019 +0.48 +.045 +.112 +.217 -.023
15000 -.023 -.63 -0.93 -.084 -.035
20000 -.069 -.112 -.185 -.149 -.239
25000 -.078 -.152 -.240 -.272 -.307
30000 -.095 -.161 -.264 -.321 -.322
Latitudes 40 & 30 Latitudes 30 & 20 Latitudes 20 & 10 Latitudes 10 & 0
Height. Wind. Vapour. Wind. Vapour. Wind. Vapour. Wind. Vapour.
0 +.854 -.082 +.648 -.132 +.447 -.109 +.208 -.082
5000 +454 -.082 +.392 -.107 +.282 -.073 +.118 -.042
15000 +.031 -.009 +.038 -.022 +.036 -.019 +.008 -.013
20000 -.131 -.024 -0.12 -.068 -.018 -.045 -.007
25000 -.196 -.128 -.074 -.054
30000 -291 -.170 -.090 -.070

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I have expressed my doubts above, whether we have sufficient data to determine the amount of the resistance which the pores of the gaseous constituents of the atmosphere offer to the passage of vapour in motion. Experiments certainly are wanting to elucidate this relation; the observations, however, of De Saussure and Dalton throw some light upon the subject. The resistance alluded to, may be regarded as two-fold; first, in connexion with the permanently-elastic fluid at rest, and Secondly, with it in motion.

With regard to the state of rest, the opposition with which vapour passes through air, is, in proportion to its density. De Saussure concluded, from his experiments, that a diminution in the density of one-third doubled the rate of evaporation.

With regard to the state of motion, a breeze in opposition to the stream of vapour, must retard its progress as much as one in the same direction favours it. Much obscurity envelopes this inquiry from the vagueness of the terms employed in denoting the velocity of the air. Mr. Dalton has determined that the rate of evaporation, in a perfect calm; being denoted by 120, that of a brisk wind is 154, and of a high wind 189. The retardation of opposing currents, of the same respective forces, may therefore be reckoned in proportion.

Some important conclusions follow from these propositions, however wanting in precision. It is impossible, in the present state of our knowledge, to determine the absolute velocity with which vapour travels under any of the circumstances mentioned; but the relative rate of different parts of the same column may be approximated. Thus;

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taking latitude 30, laid down in the last table, the current which blows in the direction of latitude 40, may be deemed high, and retards the motion of the vapour towards latitude 20 accordingly. At the height of 10,000 feet, the density of the air is reduced one-third, and the velocity is consequently doubled: to which, we must also add, that the opposing current, at the same elevation, declines in strength, whereby the force is again increased in the proportion of 189 to 154. More vapour, therefore, probably would pass at this elevation than at the surface; although its excess of elasticity is only .044 inches at the former station, and .138 inches at the latter. Whenever a deep stratum of air has had its temperature and vapour equalized, in the manner before described, it is easy to conceive that the aqueous atmosphere may travel in its upper parts with considerable velocity, in a course directly opposed to the wind at its lower. The approximation may be carried a little further, perhaps, as follows. The effect of a brisk wind, in accelerating evaporation, is equal to an increase of about three-tenths of the elasticity; that of a high wind to six-tenths. The retarding influence of the polar current, in its regular state, may therefore be apportioned to the different latitudes in Tables XXXV. and XXXVI., as follows:—From the poles to latitude 80 = 1/10 of the elasticity, to lat. 70 = 2/10, lat. 60 = 4/10, lat. 50 = 5/10, lat. 40 = 6/10 lat. 30 = 5/10 lat. 20 = 3/10, lat. 10 = 2/10, and from lat. 10 to the equator 1/10. The following Table then, represents the efficient force of the vapour in a lateral direction, calculated for the surface of the sphere, and for the altitude of one-third the density.

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TABLE XXXVIII. Shewing the efficient lateral Forte of Vapour between the Poles and Equator at the Surface of the Sphere, and at the Altitude of one-third the Density.

Latitudes 90 & 80 Latitudes 80 & 70 Latitudes 70 & 60 Latitudes 60 & 50 Latitudes 50 & 40
Height. Balance of Force. Effects of Wind and Density. Balance of Force. Effects of Wind and Density. Balance of Force. Effects of Wind and Density. Balance of Force. Effects of Wind and Density. Balance of Force. Effects of Wind and Density.
0 -.003 -.002 -.010 -.008 -.028 -.017 -.049 -.025 -.103 -.042
10000 -.001 -.002 -.004 -.008 -.008 -.017 -.012 -.026 -.023 -.045
Latitudes 40 & 30 Latitudes 30 & 20 Latitudes 20 & 10 Latitudes 10 & 0
Height. Balance of Force. Effects of Wind and Density. Balance of Force. Effects of Wind and Density. Balance of Force. Effects of Wind and Density. Balance of Force. Effects of Wind and Density.
0 -.188 -.069 -.132 -.98 -.109 -.088 -.082 -.074
10000 -.044 -.072 -.050 -.090 -.088 -.070 -.029 -.058

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I must here repeat, that these tables are not meant to impose an air of precision upon the subject, which the present state of our knowledge does not warrant; but to assist our conceptions of the general effects of so many conflicting causes. The last table will give some idea of the retardation of force, in the vapour, occasioned by the wind, at the surface of the sphere, and also of the increase of velocity occasioned by diminished pressure in the upper regions. It is easy to understand that, whenever the aërial current coincides with the direction of the vapour, the progress of the latter is accelerated in the same proportion.

The permanency of the Barometric pressure, on the surface of the sphere, is dependant, as we have seen, upon the equal balance of the aërial currents, its fluctuations have been traced to the destruction of this equipoise, by unequal and local expansions and condensations. One of the chief causes of these latter, there can be no doubt, is the increase and the decrease of the aqueous vapour, counteracting the natural progression of temperature, by the caloric evolved in its condensation: but there is another, to which no allusion has yet been made, which must necessarily be powerful in this operation. It has hitherto been supposed, for the sake of simplifying the subject, that the source of heat has been in the sphere itself; and that all the regular changes of temperature have emanated from its surface. This so far agrees with the condition of the atmosphere of the earth, with which it is our final object to identify our various hypotheses; for, while in a transparent state, the sun's rays pass

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through the air without materially affecting it, and expend their energy upon the surface of the globe. But, if the atmosphere become cloudy and opaque, the rays of heat, emanating from an external source, are in great part absorbed before they reach the surface, and an increase of temperature and elastic vapour, must take place, in the middle regions. Another source is hence derived of partial and powerful expansion.

To this we may also add, the property which the clouds possess, of preventing the radiation of heat from the surface beneath them, and the greater conducting power of damp, than of dry, air.

Amongst the literally numberless modifications of circumstances, to which an atmosphere, of the nature we have been considering, is liable, there are yet two or three, to which it will be necessary shortly to refer. The surface of the sphere, has hitherto been chiefly considered as perfectly plain, and either thoroughly dry, or every where covered with water,—we will now contemplate it as covered With water, to the extent of three fourths of its superficies, and the remaining fourth of dry earth, uneven and intersected by eminences. This intermixture of land and water, at once introduces inequalities of temperature, of a different character from those that have been hitherto considered. They arise chiefly, from the greater rapidity both of heating and cooling, in the dry surface, dependant upon the peculiar constitution of the watery element. It will not be essential to our purpose to trace them into details. As the processes by which their impressions are communicated to the

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incumbent air, are slow and gradual, they mostly affect the different columns in an equable manner; so that their influence upon the currents resolves itself into the cases which have been already proposed, of total and regular expansion. With respect to the vapour, however, the case is different. It is evident, from principles before established, that the parts of the atmosphere which are immediately over the dry spaces, will not remain free from its admixture; for the elasticity of the surrounding medium will soon supply the vacuum, The rapidity of this equalization will depend upon the mechanical obstruction of the air, being increased or diminished by an adverse or favourable wind. When once diffused over the land, it would be more subject to condensation; and the amount of precipitation must be restored from the expanse of waters.

Unevenness of surface would also tend to modify the atmosphere, in an inferior manner. Any elevation would obviously partake of the temperature due to the stratum of air, into which it rose; but the action must be reciprocal, and as the heating surface is raised to higher regions, those regions must be proportionally and unequally affected.

But these, and similar particulars, belong more especially to the history of the terrestrial atmosphere; and we are now arrived at that point where we may discontinue the synthetical process, and proceed to prove the accuracy of our inductions, by the method of analysis.

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PART IV.

Examination of the Particular Phenomena of the Atmosphere of the Earth.

THE test of our theories must be their application to meteorological observations. Of these, I shall now proceed to select a few of the most general, and the best authenticated; and, after a concise statement of each fact, will endeavour to shew how far it is reconcileable with our preceding conclusions. If the principal phenomena of this intricate branch of natural philosophy should be found to derive any elucidation from the manner of viewing them here adopted, it may be worth while hereafter to review the registered changes of different climates in a more particular manner than is consistent with the nature of this Essay.

The first fact to which I shall address myself is, that

I. The mean height of the Barometer at the level of the sea, is the same in every part of the globe.

Equality of pressure is one of the fundamental laws of Hydrostatics, and, consequently, we have seen that it is one of the first conditions of an atmosphere at rest. We have also seen, that, when acted

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upon by disturbing causes, the restoration of this equilibrium is the object of all the motions excited. Where the cause is permanent and equal, the effect produced is exactly adequate to maintain an unfluctuating balance; and equality of pressure is attained by means of regular currents. Where the cause is temporary and partial, it has not time to effect a general adjustment, and partial disturbances are the consequence; but the local effects, which are in reality mere undulations of the medium, must always be, as much in excess on one side as they are in defect on the other, and oscillate round the same point of equilibrium. The balance of fluctuations will, therefore, still exhibit equality of pressure.

II. The Barometer constantly descends in a geometrical progression for equal ascents in the atmosphere, subject to a correction for the decreasing temperature of the elevation.

This observation applies, whatever be the height of the mercurial column at the surface of the sea, and however remote from its mean state.

The density of the air is the result of the action and re-action of its gravity and elasticity; and between the two forces, there must be an exact balance, that is to say, the weight of the air which tends to compress it, and the elasticity by which it endeavours to expand, must be equal. Now the force of gravity being exerted in a perpendicular direction, any increase or decrease in the two antagonist powers must instantly pervade the whole of the perpendicular column in which it takes place; so that, under every circumstance of disturbance,

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the geometrical progression of the density will be maintained. A local change in any section of the atmosphere is thus instantly distributed throughout its total height; while the general compensation of the system is more gradually produced by lateral movements.

The method of correcting the height of the mercurial column, for the temperature, usually adopted in Barometrical mensurations, is by no means correct It consists in estimating the temperature of the air, by taking an arithmetical mean between the heights of the thermometer at the upper and lower stations, upon the supposition of the uniform diffusion of heat in the column intercepted between them. Although its adoption, in cases of moderate elevation, is attended by no very sensible error, its insufficiency is very manifest at great altitudes. M. De Luc, when he proposed this correction, was sensible of its imperfection; and General Roy observed, that "one of the chief causes of error in barometrical computations proceeds from the mode of estimating the temperature of the column of air from that of its extremities; which must be faulty, in proportion as the height and difference of temperature are great." The preceding speculations naturally suggest some ideas upon this subject, Which it would occupy too much time to attempt to develop here. It is sufficient for our present purpose, to establish that some correction for temperature is necessary.

III. The mean temperature of the earth's surface increases gradually from the poles to the equator.

Upon this fact it is not necessary to enlarge. It

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is well known to be the result of the unequal impression of the sun's rays. At the middle point the temperature has been found to be the exact mean; and from that centre the heat diminishes rapidly northwards, and increases with equal rapidity towards the south. The incumbent atmosphere of course follows the same gradation.

IV. The mean temperature of the atmosphere decreases from below upwards, in a regular gradation.

The fact is sufficiently established, by numerous observations.—Mr. Dalton was the first to demonstrate, that the natural equilibrium of heat in an atmosphere is, when each atom of air in the same perpendicular column, is possessed of the same quantity of heat; and, consequently, that such an equilibrium results, when the temperature gradually diminishes in ascending. This is the natural consequence of the increased capacity for heat, derived from rarefaction. When the quantity of heat is limited, the temperature must be regulated by the density. Professor Leslie* also, by some delicate experiments, determined the expression erf this law of progression; and has shewn that, reckoning the density of the air, at the surface of the sphere, as unit, the difference between the density at any given altitude, and its reciprocal, being multiplied by 45, will express the mean diminution of temperature in degrees of Fahrenheit's scale. This rule accords exactly with numberless experiments made in different parts of the globe. Observations, likewise, prove that the fact is only elicited from the mean results; the actual daily temperature

* Ency. Brit. Sup., Article CLIMATE, p. 188.

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being found to oscillate at certain distances on each side of this centre.

V. The Barometer y at the level of the sea, is but very slightly affected by the annual or diurnal fluctuations of temperature.

This will be apparent from every register, whose mean observations are consulted with this view. The vicissitudes of day and night, and the changes of the seasons, are produced by very gradual processes of heating, and cooling; the alterations of temperature have time to pervade the whole column, and the balance is preserved by the equalization of the effect. The heating power, in general, being the surface of the earth, the arrangement is in the most perfect form for effecting this end. The air above becomes cooled from its position, and of greater density than is due to its elevation; while that below is expanded by its contact with the heated body, and a rapid interchange of situation must necessarily ensue.

This cooling of the atmosphere by radiation, must be distinguished from that decrease of temperature, which arises from increased capacity. The amount of the latter is definite for the height, and it is a constituent element of the density due to the elevation; but by the former power, the air is cooled below this standard, and becomes in its successive strata specifically heavier. That this emission of heat into space is constantly going on, is evident from the permanency of the mean temperature, which, notwithstanding the constant

H

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accession which it receives from the sun, ever remains the same.

But although any undue acceleration of either of the great principal currents is prevented by this mixture to any extent, yet, in most parts of the globe, a tendency has been observed in the Barometer to fall during the day, and to rise at night*; which would seem to indicate that the lower stratum becomes rather more affected by the diurnal temperature than the upper. This exception, however, is perfectly reconcileable to theory and to observation; for it is well known, that the surface of the earth becomes more rapidly cooled by radiation, during the absence of the sun, than the air; a small stratum, therefore, which is in immediate contact with it, becomes unduly affected; and, as the influence, from its nature, cannot be extended upwards, an increase in the weight of the whole column takes place. The returning heat dissipates this accession by restoring the natural progression of temperature.

VI. The Barometer, in the higher regions of the atmosphere, is greatly affected by the annual and diurnal fluctuations of temperature.

This observation is easily confirmed in various ways; but for the present, I shall refer for its correctness to those valuable registers, which are simultaneously kept at Geneva, and the summit of Mont St. Bernard. Upon the average of four

* See the Essay upon the Horary Oscillations of the Barometer.

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years, at. those stations, I find, that from sun-rise to 2 P.M., the upper Barometer gains upon the lower .037 inch, and from winter to summer .260 inch. This will be deemed direct and satisfactory proof of the proposition, although the places where the observations were made, are not exactly suited to exhibit the fact in its most striking form. The city of Geneva is situated at a great height above the level of the sea, and must, therefore, itself partake of the barometric change referred to. To this we may add, that experiments, made upon elevations of the surface of the earth, can by no means be looked upon exactly in the same light, as if they had been performed at equal elevations, in the atmosphere above the level of the sea. In the former case, the heating surface is raised into the higher regions, and cannot fail to produce a modification of circumstances, different from those which the simple problem supposes. As the existence, however, of an effect, and not its amount, is heife required to be proved, the example will suffice.

It is a consequence which naturally, and infallibly, flows from that general alteration of temperature, which does not affect the Barometer at the level of the sea: for as the expansion and contraction do not alter the total weight of the aërial column, it is clear that they must change the relative weights of its different sections.

VII. The heating and cooling of the atmosphere, by the changes of day and night, take place equally throughout its mass.

This is fully established by the same series of

H 2

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observations. Some apparent exceptions will be found, upon close examination, to confirm our conclusions. The mean ranges of the thermometers, for the twenty-four hours, exactly correspond, during the winter months, in the two situations: but in the summer, the lower exceeds the higher, 5 or 6 degrees. The reason of this discrepancy will be obvious, when we recollect that the convent of St. Bernard is situated within the influence of perpetual snows: much of the rising heat is, therefore, expended in their liquefaction, in the months when the daily temperature rises above the term of congelation. This is a striking example of the difference, which we have before referred to, between observations made upon lofty stations of the earth's surface, and at equal altitudes in the atmosphere above the sea. The impression of the surface, whether it be a heating or cooling power, must be communicated to the surrounding air.

This influence may also be traced in another way, and presents an illustration of a different point. The cooling power of the snow must produce an unequal effect upon the currents; and accordingly we find that a proportionate effect is indicated by the Barometer, at the base of the column, which stands a little higher in the summer months than in the winter.

VIII. The average quantity of vapour in the atmosphere decreases from below upwards, and from the equator to the poles.

This consequence is obviously derivable from the

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preceding laws of temperature, and is, moreover, amply confirmed by experiment.

IX. The condensation of elastic vapour into cloud raises the temperature of the air.

In confirmation of this theoretical and practical conclusion, the following observation of M. de Luc may be adduced.

"Pendant que je réfléchissois sur l'apparition subite des nuages, je découvris un petit amas de vapeurs, du côté du nord, à 3 ou 400 pieds au-dessous de moi: Je le considérais avec attention, et je remarquois d'abord que son volume augmentoit sensiblement, sans qu'il me fût possible d'ap-percevoir d'où lui venoient ses accroissements. Je vis ensuite qu'au lieu de s'abaisser à mesure qu'il grossissoit, et qu'il paroissoit même devenir plus dense, il s'èlevoit au contraire. Le vent le pous-soit vers moi. Il m'atteignit enfin, et m'environna tellement que je ne vis plus ni le ciel ni la plaine. Je pensai au même instant, à observer mon thermomètre, qui étoit suspendu en plein air, éxposé au soleil et que j'avois vu auparavant à + 4⅔(42° Fah.). Je présumois que l'action du soleil étant interceptée par ce nuage mon thermomètre devoit baisser et je fus très surpris de le voir au contraire à + 5½ (45° Fah.). Le nuage, qui continuoit à monter obliquement vers le sud, abandonna bientôt le lieu où j'étois, le soleil reparut mais, malgré son action, le thermomètre redescendit." DE LUC, tom. iii., p. 251.

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X. Another remarkable phenomenon is, that there exists a general tendency in the wind to blow from north-east and south-east towards the equator, in latitudes below 30°.

The main-spring of all the grand movements in the atmosphere is, no doubt, the regular gradation of temperature, which exists along the different latitudes of the earth's surface. It was Hadley who first attributed to its right cause, viz., the excess of the rotatory velocity of the equator, the flexure towards the west, which the great polar currents receive, and which is known by the name of the trade-winds*. These winds, as they approach the equator, gradually lose their northerly and southerly directions, and blow directly from the east. This may be explained by the meeting of the currents from the two poles, which have thus their opposite impulses balanced; when nothing remains, but that excess of inertia which leaves them behind the revolution of the globe. For the same reason, they lose much of their energy in this situation, and the neutral line is subject to frequent calms.

XI. While the trade wind blows upon the surface of the earth, a current flows in the contrary direction, at a great elevation in the atmosphere.

This necessary consequence of the theory of the trade winds, rested for a long time upon theoretical conclusions only: the eruption, however, of the volcano, in the Island of St. Vincent, in the year

* Phil. Trans. 1735.

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1812, placed the fact beyond dispute. The Island of Barbadoes is situated considerably to the east of St. Vincent, and, between the two, the trade wind continually blows, and with such force, that it is with considerable difficulty, and only by making a very long circuit, that a ship can sail from the latter to the former. Notwithstanding this, during the eruption at St. Vincent, dense clouds were formed at a great height in the atmosphere above Barbadoes, and a vast profusion of ashes fell upon the island. This apparent transportation of matter against the wind, caused the utmost astonishment amongst the inhabitants, and the certainty of the fact cannot but be considered as of the utmost interest to the science of meteorology.

XII. The mean height of the Barometer is not affected by the trade winds.

This is a proof that the quantity of air, which passes below from the poles to the equator, must be exactly balanced by an equal quantity flowing above in the opposite direction.

XIII. Between the latitudes 30° and 40°, both in the northern and southern hemispheres, westerly winds prevail.

The atmosphere, from the processes of heating and cooling, which it is incessantly undergoing, and from the position of the heating surface, must, as we have seen, be subject to a constant circulation from below upwards; but the direction of the falling particles is diverted from the exact perpendicular direction by the influence of the great lateral cur-

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rents: they, therefore, reach the earth in an angular course. Thus, the molecules, situated over the equator, which become cooled by their remoteness from the earth, descend from their increase of specific gravity; but in their descent are carried by the set of the superior stream towards either tropic. Bearing with them, as they do, the excess of the equatorial velocity, from west to east, they no sooner enter the lower current than they impart this movement to it, and the wind is modified accordingly. The spaces included between latitudes 30° and 40°, appear, from observation, to be the regions where this influence first takes effect, and primarily prevails. If it be asked, why this effect, depending apparently upon as constant a cause as the trade winds themselves, is less certain and steady in its occurrence; the answer will lead us to other causes, which affect both it, and sometimes the eastern winds, and which will presently come under our notice. We may add to our remarks, at present, that the westerly winds, within the above mentioned limits, are much more regular and constant in the southern hemisphere than in the northern. It may further be observed, that the restriction itself of the east winds, within the 30th degree of latitude, is owing to this counteracting influence, and that the strictness of their limits can be explained upon no other hypothesis.

XIV. The western coasts of the extra-tropical continents have a much higher mean temperature than the eastern coasts.

This difference is extremely striking between

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the western coast of North America and the opposite eastern coast of Asia. It is explained by the heat evolved in the condensation of vapour, swept from the surface of the ocean by the western winds. This general current, in its passage over the land, deposits more and more of its aqueous particles, and by the time that it arrives upon the eastern coasts, is extremely dry: as it moves onwards, it bears before it the humid atmosphere of the intermediate seas, and arrives upon the opposite shores in a state of saturation. Great part of the vapour is there at once precipitated, and the temperature of the climate raised by the evolution of its latent heat.

XV. A wind generally sets from the sea to the land during the day, and from the land to the sea during the night, especially in hot climates.

The land and sea-breezes are amongst the most constant of the phenomena of the inconstant subject with which we are occupied. The land becomes much more heated by the action of the sun's rays than the adjacent water; and the incumbent atmosphere is proportionably rarefied: during the day, therefore, the denser air of the ocean rushes to displace that of the land. At night, on the contrary, the deep water cools much more slowly than the land, and the reverse action takes place. As these changes proceed gradually, the height of the barometer is not affected by them.

XVI. The trade winds, in the neighbourhood of the western coasts of the large continents, in their course, have their direction changed.

This is an effect of the same nature as that of the

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land and sea-breezes. Those parts of Africa and America which lie between the tropics, become intensely heated by the action of a vertical sun: the columns of the atmosphere, which rest upon them, must therefore be highly rarefied, and the more temperate air of the surrounding seas will press upon them. This influence is so decided as to overcome the tendency of the east wind; and on the western coasts of both continents a wind from the west prevails. This is, again, an instance of a complete perpendicular change from a permanent cause, and the total pressure is unaffected.

Of the same nature are the Monsoons of the Indian Ocean, and other periodical winds. They are occasioned by a particular distribution of land and water, acted upon by the periodical changes of the sun's declination. While the sun is vertical to the places where they occur, the land becomes heated, and the air expanded, and the wind flows toward the coasts. As the sun retires towards the opposite point of its course, the land cools faster than the surrounding seas, and the course of the winds is westward. The simplest way of regarding the sun's motion in declination, as affecting the temperature of the various latitudes, is, to suppose a motion of the whole system; by which the line of greatest heat, and the two points of greatest cold, maintaining their relative distances, vibrate on either side of the earth's equator and poles.

None of these changes affect the barometer.

It may now be understood, that it is the intermixture of land and water, joined with other disturbing causes, which prevents the extra-tropical

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western winds from being as regular in their course as the tropical trade winds, and a slight inspection of the chart will demonstrate, that the northern hemisphere, including the great continents of Europe, Asia, and North America, must be more under this influence than the southern, which is comparatively free from such effects.

XVII. Rain seldom occurs in the constant trade winds, out abundantly and constantly in the adjoining latitudes.

Between the tropics, the elasticity of the aqueous vapour reaches its maximum amount, and within these limits only, rises to any extent into the upper current of the atmosphere. Its own force, therefore, which is laterally exerted, is assisted by the equatorial wind, and it flows to the north and south as fast as it rises within the zone. No accumulation can, therefore, be formed; and the temperature being remarkably steady, seldom varying more than two or three degrees, precipitation can but seldom occur.

The continental parts, however, of the same regions, being liable to greater vicissitudes of heat, are subject to rainy seasons, which are periodical, like the Monsoons of the same climates, and are governed, as they are, by the progress of the sun in declination. The condensation, while it lasts, is in proportion to the density of the vapour, and is violent beyond any thing that is known in temperate climates. The alternate seasons of fine weather are distinguished by cloudless skies and perfect serenity.

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The extra-tropical latitudes, on the contrary, beyond the bounds of the trade winds, are, at all times, exposed to great precipitations. The vapour in its course is subjected to a rapidly decreasing temperature, and the condensation is fed by a constant supply. We are thus led to the consideration of a temperate zone, and a variable climate.

XVIII. Between the tropics the fluctuations of the Barometer do not much exceed ¼ of an inch, while beyond this space they reach to 3 inches.

The great characteristics of the tropical regions are constancy of temperature, and freedom from aqueous precipitation. I speak now more particularly of the included oceans, which are so extensive, compared with the land, as necessarily to stamp the character of the climate. The phenomena, on the latter, are so limited as scarcely to affect the total result, and are to be regarded more in the light of exceptions, in whatever points they differ from the general rule.

Now variations of temperature alone have been satisfactorily proved not to affect the mercurial column: and it is in the aqueous condensation, that we shall probably find the cause of barometrical changes. The vapour, as we have seen, passes north and south from the equatorial parts, and reaching the extra-tropical regions, is precipitated. The effect of this precipitation must be to destroy the progression of temperature in the vertical columns, by equalizing the heat of the strata exposed to its influence. But, as this process is carried on chiefly in the lower current,

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and cannot, from its very nature, equally affect the column, the total weight will be reduced by the consequent irregular expansion.

XIX. In the temperate climates the rains and the winds are variable.

One of the great causes, of the variableness of the wind, has already been pointed out in the greater abundance of land in the extra-tropical parts of the northern hemisphere. Another cause obviously must originate in the variations of barometrical pressure. The rain must depend very much upon the changes of the wind, and the retardation or acceleration which they offer to the progress of the vapour. But another cause arises from the unequal supply, which the process of evaporation receives from the irregular surface of the globe. This cannot be placed in a stronger light than by the following considerations. The Caspian Sea, which is placed in the centre of the largest continent of the world, receives the precipitations of an immense tract of the atmosphere by means of the rivers which flow into it, and drain the neighbouring countries. The whole of this supply is again returned by evaporation, and its waters have no other means of escape. The lakes of North America, situated in nearly the same parallel of latitude, and at the same altitude, receive the drains of a much less space; but annually roll an immense volume of water to the ocean. We are thus furnished with an hygrometer upon a large scale, by which we may judge of the state of saturation of the two atmospheres. The

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difference can arise from no other cause than the proximity of the surrounding seas in the latter situation, which furnish an inexhaustible source of vapour, which is deficient in the other. It is for the same reason that less vapour is contained in the atmosphere above a continent, than above the ocean, although more rain falls in the former situation than in the latter under the same latitudes, owing to the greater vicissitudes of temperature. Much of the aqueous atmosphere which is formed from the great deeps, is thus drawn off towards the continents, where a scarcity of water occasions an inadequate pressure of the vapour.

XX. As we advance towards the Polar Regions we find the irregularities of the wind increased; and storms and cairns repeatedly alternate, without warning or progression.

The very instructive "Account of the Arctic Regions," for which we are indebted to Captain Scoresby, and the interesting Journal of Captain Parry, have made us well acquainted with the interesting regions of perpetual ice and snow. In our hypothetical statement of the progression of the earth's temperature, we have supposed no greater cold to prevail than that of 0° at the poles, but the experience of our intrepid Navigators has proved that a cold of 50° greater intensity sometimes prevails in latitudes still far removed from the 90th degree. The density of the air is, of course, proportionately increased, and its sources of inequality multiplied. When the sun is above the horizon, it

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produces comparatively little effect upon the icy mountains, while the neighbouring seas are warmed by its unceasing influence. The extremes of heat and cold will sometimes prevail within a very limited compass; and forcible winds will blow in one place, when, at a distance of a few leagues, gentle breezes prevail. "Ships, within the circle of the horizon, may be seen enduring every variety of wind and weather, at the same moment; some under close-reefed top-sails, labouring under the force of a storm; some becalmed and tossing about by the violence of the waves; and others plying under gentle breezes from quarters as diverse as the cardinal points." The fluctuations of the barometer are also great and sudden, proving what theory would have induced us to conclude, that the irregularities of these regions extend to the higher strata of the atmosphere.

XXI. In the extra-tropical climates, a fall in the Barometer almost always precedes a period of rain, and indicates an acceleration or change of the aërial currents.

As the proximate cause of the fall of the barometer is an accumulation of aqueous vapour, and a consequent unequal expansion of the atmospheric columns, it is obvious that this alone would increase the probability of a proportionate precipitation; but it is not the only reason of the effect. The fall of the barometer indicates a decrease of density in the aërial currents, and, consequently, a decrease of the resistance to the passage of the vapour. A constant stream will thus rush in with increasing force,

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augmenting by its condensation the cause of its velocity, till a current sets in from some other quarter, and restores the equilibrium.

XXII. Barometers, situated at great distances from each other, often rise and fall together with great regularity.

This proves that the cause of the variations must be very extended in its influence; and from all our preceding inquiries the conclusion à priori would be the same. The proximate cause is one of unlimited extent, and in so fluid a medium the remoter influence must be widely felt. It has been observed that this unison of action extends further in the direction of the latitude than in that of the longitude, and the remark materially confirms our theory; for, as the grand currents of the atmosphere flow in the direction of the meridians, any irregularity in their courses would most readily be propagated in the same line.

XXIII. More than two currents may often be traced in the atmosphere at one time by the motions of the clouds, &c.

The great fluctuations of the atmosphere have been referred to modifications and disturbances in the courses of two principal currents, but from the very nature of the disturbing causes which we have been considering, it is obvious that these must often include subordinate currents and minor systems of compensation within themselves. When we recollect, not only that every mountain and sea, but that every hill, and lake, and river, (not to descend to the more minute and numberless influences of artificial

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arrangements,) produce very appreciable impressions, it is obvious that we must consider the great streams as made up of inferior circulations; which, like the eddies of a river, or the waves of the sea, do not affect the main flux of the currents.

XXIV. The force of the winds does not always decrease as the elevation increases; but, on the contrary, is often found to augment rapidly.

A slight inspection of Table VIII. will shew that this remark is in exact correspondence with the theory. It will there be seen that, in the regular course of the currents, the lower wind dies away gradually as we ascend, and at a certain height gives place to a gentle breeze in a contrary direction: this increases in force with the height, till, notwithstanding the rarity of the air, its impulse is so great as to produce a very strong wind. Some of the hypothetical cases, of the disturbance of this regular order, present still more marked instances of winds increasing upwards.

XXV. The variations of the Barometer are less, in high situations, than in those at the level of the sea.

The range of the barometer for any given latitude, should be in inverse proportion to its elevation in the atmosphere: for, as we have seen, under all circumstances, the law of the progression of density must be maintained. Any cause, therefore, producing a given change at an altitude whose density is one half, must produce double the effect at the level of the full pressure; and, on the other hand, a rise of the barometer, of two-tenths of an inch at the

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base of a column, will only be felt as one-tenth at the height due to half the density.

The British Islands are situated in such a manner as to be subject to all the circumstances which can possibly be supposed to render a climate irregular and variable. Placed nearly in the centre of the temperate zone, where the range of temperature is very great, their atmosphere is subject, on one side, to the impressions of the largest continent of the world, and, on the other, to those of the vast Atlantic Ocean. Upon their coasts the great stream of aqueous vapour, perpetually rising from the western waters, first receives the influence of the land, whence emanate those condensations and expansions which deflect and reverse the grand system of equipoised currents. They are also within the reach of the frigorifie effects of the immense barriers and fields of ice, which, when the shifting position of the sun advances the tropical climate towards the northern pole, counteract its energy, and present a condensing surface of immense extent to the increasing elasticity of the aqueous atmosphere. Amidst all the uncertainty and seeming confusion arising from this complication, our general principles may still be recognised; and I would fain hope that the more they are btadied, the more obvious they will appear, as in this case, above all others, "exceptions prove the rule.'

XXVI. In Great Britain, upon an average of ten years, westerly winds exceed the easterly in the proportion of 225 to 140.

From the geographical situation of the island, we

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should be led to suppose that the atmosphere must come within the influence of the descending equatorial particles, and that winds from the west would prevail; and this conclusion is confirmed by the observation. Of those from the east, the northerly exceed the southerly in the proportion of about 74 to 54; leaving but a very small proportion indeed, which blow from the most irregular point, viz., the south-east. The north-east may be regarded as those which, coming from the north, have not attained the velocity of rotation due to the latitude.

XXVII. Upon the same average, the northerly winds are to the southerly as 192 to 173.

By this classification we most readily detect the influence of local disturbing causes. Were it not for these, the northerly current would prevail throughout the year: but the condensation of vapour to the north and north-east of this situation is so great as to cause perpetual diminutions of the aërial columns, and consequent deflections of the currents. In the central parts of Europe, the northern winds are much more regular; and there, especially in summer, the Etesian breeze constantly prevails. Of the winds from the south, it may also be remarked, that the westerly exceed thq easterly in the proportion of 104 to 54.

XXVIII. Northerly winds almost invariably raise the Barometer; while southerly winds as constantly depress it.

The northerly current is the natural couise of the

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air, and, where the regular order has been disturbed; in consequence of diminished local pressure, a .return to it restores the equilibrium. Coming from the frozen arctic regions, it speedily reduces the accumulation of vapour, stops the supply, and dissipates the concomitant heat, from which originated the depression; and, if it flow with a velocity beyond its regular rate, causes a reduction of temperature below the due progression, and augments the total weight. The southerly wind, on the contrary, facilitates the passage of the vapour, which by its unremitting condensation unceasingly increases the cause of depression.

XXIX. The most permanent rains of this climate come from the southern regions.

The supply of vapour, which occasions rain, may be traced to two sources:—One is the evaporation of the latitude itself, where it is precipitated; and the other, the stream which is perpetually struggling to advance from the equatorial zone. These causes sometimes act conjointly, and sometimes separately. Rain from the first, is derived from sudden falls of temperature, produced by cold currents, or the changes of the seasons, and assumes the form chiefly of showers of greater or less continuance. The expenditure of vapour is but slowly supplied, and the precipitation occurs at intervals. Rain, from this source, is always accompanied by a declining temperature. When, on the contrary, in consequence of diminished pressure, the tropical current reaches in succession the colder parallels, the supply continues in a perpetual flow; the tem-

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perature is raised, the depression of the Barometer increases, and rain descends with little intermission.

XXX. The mean height of the Barometer varies but little with the changes of the seasons.

Many persons have supposed that the fluctuations of the Barometer are owing to the greater or less weight of the aqueous particles, contained in the atmosphere, at one time than at another. If, however, our theory be correct, the difference of pressure between a perfectly dry atmosphere, and one saturated with moisture, cannot much exceed .150 inch: the difference of the seasons must, therefore, be even less than this amount. But, small as it is, it may nevertheless be detected by the system of averages. Mr. Howard, in his invaluable work, upon the climate of London, has brought out the difference of pressure, for the several seasons, upon a calculation of ten years' observations, as follows:—

Brumal period above the autumnal .021 inch, Vernal period, above the Brumal, .030 inch, Estival, above the Vernal, .045 inch, and Autumnal below the Estival, .096 inch. These results are in exact accordance with our theoretical conclusions.

The subject of atmospheric vapour has hitherto been less studied than its importance would seem to require. Observations upon its variations are very deficient, owing to the uncertainty and imperfection of the means generally adopted for measuring its effects. A few general conclusions which

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have suggested themselves to me, from the constant use for three years of a more perfect instrument for that purpose, and some valuable results derived from the experiments of my friend, Captain Sabine, during a twelve-months' residence between the Tropics, will add no small weight to our synthetic deductions.

XXXI. The elasticity of the aqueous vapour does not decrease gradually as. we ascend in the atmosphere, in proportion to the graduai decrease of the temperature and density of the air; but the dew-point remains stationary to great heights, and then suddenly falls to a large amount.

To this conclusion I had been led, by my theoretical speculations, long before I had any hopes of being able to confirm it, by direct experiment. I have, however, at last succeeded in obtaining complete evidence of its correctness. As the fact is altogether new in Natural History, and as ft is essential to the theory which I have been endeavouring to establish, forming indeed the test by which it may be most correctly judged, I shall, I trust, be excused for enlarging somewhat upon it.

The first experiments to which I shall refer, as bearing upon this point, are those of Mr. Green, the Aëronaut. An account of this gentleman's ascent in a Balloon, from Portsea, on the 6th of September, 1821, is given in the 12th volume of the Journal of the Royal Institution (p. 114). Amongst other instruments of research, he took up with him one

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of my Hygrometers. He unfortunately omitted to take the point of deposition, before he commenced his ascent; but the omission is of less consequence as I happened to make an observation at the time, at no very great distance from the spot. At an elevation of about 9890 feet, he found the dew-point at 64°, exactly the same as I ascertained it to be at the surface of the earth. At 11060 feet it had fallen to 32°, making a difference of 32 degrees in little more than 1100 feet. Here, then, we have presumptive evidence of an immense bed of vapour rising in its circumambient medium, unaffected by decrease of density or temperature till checked by its point of precipitation; and of an incumbent bed of not much more than one-third the density, regulated, no doubt, as the last, by its own point of deposition in loftier regions.

Captain Sabine, by his experiments upon mountains in tropical climates, has established the same fact in the most unexceptionable manner. At Sierra Leone, he ascertained that the dew-point of the vapour, at the level of the sea, was 70°; and that it was the same at the same hour upon the summit of the Sugar-loaf Mountain, 2520 feet above. At the Island of Ascension, the barometer, 17 feet above the level of the sea, stood at 30.165 inches—temperature of air 83°, and the dew-point 68°. On the summit of the mountain the barometer fell to 27.950 indies, and the temperature of the air to 70°, while the dew-point only declined to 66°. 5; so that in a height of 2220 feet, the temperature of the air fell 13°, and the constituent temperature of the vapour 1°.5.

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At Trinidad, the temperature of the air at the level of the sea was 82°, and the dew-point 77°; 1060 feet above, they were both 76.°5, and precipitation was going on.

At Jamaica, by the sea-side, the temperature of the air was 80°, and the point of deposition 73°; while, on the mountains, at a height of 4080 feet, they were both 68°.5. At a station, not five hundred feet higher, by experiment twice repeated, the point of deposition was found to be 49°, and the temperature of the air 65°.

These results are utterly irreconcileable with the idea of the aqueous particles in the atmosphere being suspended by any law analogous to that of chemical solution; and I am much mistaken if they may not be received as experimental confirmation of the theory of mechanical mixture.

Captain Sabine's experiments furnish also some evidence of that slight diminution of density in the upper parts of the beds of vapour which would arise from the decrease of their own pressure, and which I have anticipated in the second part of this essay. In the experiments at Jamaica, the dew-point fell about 4°.5 in 4080 feet. In Table XXIII., it will be seen, that I have calculated this diminution to be 3°.5 in 5000 feet, for an atmosphere of much less density.

In noticing here the demonstrative nature of the evidence in favour of the mechanical theory of the mixture of the gases with vapour, I cannot refrain from saying a few words with regard to the same theory as applied to the mixture of gases with one another. With a view of simplifying as much as

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possible a subject which, above all others, perhaps, is the most complicated in nature, I have spoken of the permanently-elastic fluid of the atmosphere as a simple gas, whereas, it is well known, to be compounded of two or three. The constancy of the proportions, in which these are found to be combined in every situation, is the never-failing theme of wonder and admiration, and it is perpetually referred to a3 evidence of chemical combination. I must own, that it strikes my mind in a very different manner, and I conceive that the fact is much more reconcileable to the mechanical than to the chemical theory.

If we suppose a consumption of the oxygen to take place, by the decomposition of the atmosphere, at any given spot, in what way is chemical affinity to act to restore the uniformity of the compound? No evolution of oxygen takes place, and it cannot be supplied by the surrounding portions; for the affinity of nitrogen for oxygen can never be supplied by the decomposition of nitrogen and oxygen, which are held together by the same affinity. On the other hand, if the oxygen and the nitrogen be two distinct elastic atmospheres, mutually permeating and pervading each other's interstices, the particles of each pressing only upon like particles, and only slightly opposing one another in their separate motions, then a local consumption of oxygen would be instantly supplied by a rush of the elastic fluid towards the spot where the equality of pressure had deen disturbed. In the same way, any partial supply of either gas would instantly be equalized; and that equal diffusion which is avowedly inexplicable

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upon chemical principles, is perfectly reconcileable to the principles of hydrostatics. I shall not now stop to insist upon the analogy between vapours and gasses, though the late beautiful experiments of Mr. Faraday have almost annihilated the distinction even in name; but shall revert to the subject, from which it may be thought that I have been wandering.

XXXII. The tension of vapour given off in the process of evaporation is determined, not by the temperature of the evaporating surface, but by the elasticity of the aqueous atmosphere already existing.

I have often endeavoured, by means of the hygrometer, to detect, within a limited circle, a difference in the elastic state of the vapour incumbent upon different surfaces of various temperatures, but without success: the rising vapour was always of the same quality, whether from water, vegetation, or ploughed land; in sun-shine, or in shade. For the same reason, the dew-point is but little affected by the increase of daily temperature from morning to afternoon, or by its subsequent declension at night. But one of the most remarkable confirmations of the fact was ascertained by Captain Sabine upon the coast of Africa. While the sea-breeze was blowing upon that station, the hygrometer denoted the dew-point to be about 60°, but when the wind blew strong from the land, it approached, in its characters, to a Harmattan; and the point of precipitation was not higher than 37°. 5, the temperature of the air being 66°. Notwithstanding the heat of the evaporating surfaces in the interior of that con-

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tinent, the burning sands of its deserts yield so little vapour, that it becomes attenuated by its diffusion, and there can be little doubt that the aqueous atmosphere incumbent upon it, and which, when wafted to the coast by the rapid motion of the air, constitutes the true Harmattan, is not of greater force than that which rests upon the polar seas; and that While the heat of the air sometimes approaches to 90°, the constituent temperature of the vapour is below 32°.

XXXIII. The apparent permanency and stationary aspect of a cloud is often an optical deception, arising from the solution of moisture on one side of a given point, as it is precipitated on the other.

No phenomenon is more common amongst mountain, or upon hills by the sea-side, than clouds upon the summits which appear to be perfectly immoveable, although a strong wind is blowing upon them at the time. That this should be the real state of the case, is clearly impossible, as so attenuated a body as constitutes the substances of the clouds must obey the impulse of the air. The real fact is, that the vapour, which is wafted by the wind, is precipitated by the cold contact of the mountain; and is urged forward in its course till, borne beyond the influence which caused its condensation, it is again exhaled and disappears. A slight inspection and consideration of the phenomena will be sufficient to convince any one of the correctness of this explanation. Reasoning from analogy, we may conclude, that the process which thus proceeds, under our eyes, upon the summits of

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the hills, likewise takes place on either side of the planes of precipitation in the heights of the atmosphere: the vapour is continually condensed, as continually re-dissolved in the act of precipitation, and the cloud appears to be unchanged and stationary.

XXXIV. The quantity of vapour in the atmosphere in the different seasons of the year (measured on the surface of the earth, and near the level of the sea,) follows the progress of the mean temperature.

This result of observation might readily have been anticipated; for the rate of evaporation, and the quantity which the air can support, are both obviously dependent upon the same progression, But this connexion is not discoverable in short periods; and the changes of diurnal temperature do not materially affect the quantity of elastic vapour. The air, at night, generally readies the point of deposition, even at the surface of the sea, but in a very gradual manner: and at the same time the supply from evaporation ceases. The progress of the vapour in fine weather may often be very satisfactorily traced by means of the clouds. During the heat of the day it rises from the surface of the land and waters, and reaches its point of condensation in greater or less quantities at different altitudes. Partial clouds are formed in different parallel planes, which always maintain their relative distances. The denser forms of the lower strata, as they float along with the wind, shew the greater abundance of the precipitation at the first point of deposition, while the feathery shapes and lighter texture of the upper, attest a rarer atmosphere. These clouds do

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not increase beyond a certain point, but often remain stationary in quantity and figure for many hours: but as the heat declines they gradually melt away; till at length, when the sun has sunk below the horizon, the ether is unspotted and transparent. The stars shine through the night with undimmed lustre, and the sun rises in the morning in its brightest splendour. The clouds again begin to form, increase to a certain limit, and vanish with the evening shades. This gradation of changes, which we so often see repeated in our finest seasons, might, at first, appear to be contrary to our principles; and that precipitations should occur with the increase of temperature, and disappear with its' decline, would seem, at first sight, to be diametrically opposite to all our conclusions. But a little consideration will shew that these facts confirm our theory. The vapour rises and is condensed; but in its precipitation fells into a warmer air, where it again assumes the elastic form; and as the quantity of evaporation below is exactly equal to supply this process above, the cloud neither augments nor decreases. When the sun declines, the surface of the earth cools more rapidly than the air; evaporation decreases, but the dissolution of the cloud continues. The supply at length totally ceases, and the concrete vapour melts completely away. The morning sun revives the exhalations of the earth, and the process of nimbification again commences, and again undergoes the same series of changes. The fall of the temperature shifts a little the planes of deposition, but scarcely affects the total pressure of the

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vapour. The deposition of dew (the formation and concomitant circumstances of which have been so successfully analyzed in the elegant essay of the late Dr. Wells,) slightly diminishes the quantity; but the first touch of the sun's rays restores it to the "blue expanse."

When, however, the natural equilibrium has been disturbed, when the temperature of the air has become equalized through various successive strata by the beds of vapour with which they are embued, the decline of the day will often detemine precipitation, and will increase its amount if already established. The result of experiment has also shewn that a greater amount of rain falls while the sun is below, than while it is above, the horizon.

XXXV. The pressure of the aqueous atmosphere, separated from that of the aërial, generally exhibits directly opposite changes to the latter.

As the quantity of vapour increases, it will mostly be found that the barometer falls; and it rises with its decrease. This observation, which is amply confirmed by tracing the lines of each upon a graduated paper, does not apply to the averages of the different seasons, but to the daily fluctuations. This fact, so utterly irreconcileable to the hypothesis which ascribes the rise and fall of the mercurial column to the weight of the aqueous particles, materially confirms that which attributes them to the unequal expansion of balancing currents. The prime source of this expansion we have supposed to be the elastic vapour, and this experience confirms the theory.

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XXXVI. Great falls of the Barometer are generally accompanied by a temperature above the mean for the season; and great rises by one below the same.

This is a confirmation of the same nature as the last, and inseparably connected with it. It is by the evolution of heat that the vapour principally acts. The mean temperature which balances all irregularities, must be the regular temperature of the climate, and cæteris paribus, that at which the currents must be most disposed to regularity: variations, on either side of this point, must produce corresponding retardations and accelerations; and these, if not general through the mass, annihilate the equipoise.

I have thus filled up the outline which I laid down for my inquiry, and I trust that it will be found that I have not wholly failed to elucidate some hitherto-obscure points of the history of the atmosphere. It tends to give me some confidence in the justness of my views, that when I first conceived the idea of conducting my researches synthetically, I anticipated but few of the conclusions to which the experiment has led me.

The principles which I have employed are the fruits of the researches of the most eminent philosophers; to have owned my obligations to whom would have loaded this essay with references. Their labours are become the foundation-stones of science, and the common property of those who may follow them in endeavours to perfect the edifice. There is one name, however, to which this branch of science

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is so vastly indebted, that to allude to it more particularly I feel to be an incumbent duty. It is that of the author of the "Essays upon the Constitution of mixed Gases." To Mr. Dalton we owe nearly, all the light which we possess upon this interesting and difficult subject; and from his deep researches, particularly, I have drawn most freely. If, indeed, these my endeavours shall be found to be deserving of any consideration, it will be as illustrations of the Daltonian theory.

I have scrupulously adhered to the natural consequences of the premises which I have adopted, without previously inquiring how far they were consonant with the phenomena to be explained in their after application to these latter, I hope that it will be found that I have not been unsuccessful, The fluctuations of the barometer, and most of the phenomena of wind and rain, appear to me to adapt themselves most happily to the theoretical conclusions.

Both the synthetical and analytical processes agree in the same grand conclusions, which may thus briefly be recapitulated:—

There are two distinct atmospheres, mechanically mixed, surrounding the earth; whose relations to heat are different, and whose states of equilibrium, considering them as enveloping a sphere of unequal temperature, are incompatible with each other. The first is a permanently-elastic fluid, expansible in an arithmetical progression by equal increments of heat, decreasing in density and temperature according to fixed ratios, as it recedes from the surface, and whose equipoise under such

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circumstances, would be maintained by a regular system of antagonist currents. The second is an elastic fluid, condensible by cold with evolution of caloric; increasing in force in geometrical progression with equal augmentations of temperature; permeating the former and moving in its interstices, as a spring of water flows through a sand-rock. When in a state of motion this intestine filtration is redarded by the inertia of the gaseous medium, but in a state of rest the particles press only upon those of their own kind. The density and temperature of this fluid have a tendency likewise to decrease, as its distance from the surface augments; but by a less rapid rate than that of the former. Its equipoise would be maintained by the adaptation of the upper parts of the medium, in which it moves, to the progression of its temperature, and by a current flowing from the hotter parts of the globe to the colder. Constant evaporation on the line of greatest heat and unceasing precipitation, at every other situation would be the necessary accompaniments of this balance. Now the conditions of these two states of equilibrium, to which, by the laws of Hydrostatics, each fluid must be perpetually pressing, are essentially opposed to each other. The vapour or condensible elastic fluid is forced to ascend in a medium, whose heat decreases much more rapidly than its own natural rate; and it is therefore condensed and precipitated in the upper regions. Its latent caloric is evolved by the condensation, and communicated to the air; and it thus tends, to equalize the temperature of the medium in which it moves, and to constrain it to its own

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law, This process must evidently disturb the equillibrium of the permanently-elastic fluid, by interfering with that definite state of temperature and density which is essential to its maintenance, The system of currents is unequally affected by the unequal expansion; and the irregularity is extended, by their influence, much beyond the sphere of the primary disturbance. The decrease of this elastieity above, is accompanied by an extremely important re-action upon the body of vapour itself: being forced to accommodate itself to the circumstances of the medium in which it moves, its own law of density can only be maintained by a corresponding decrease of force below the point of condensation; so that the temperature of the air, at the surface of the globe, is far from the term of saturation; and the current of vapour, which moves from the hottest to the coldest points, penetrate from the equator to the poles, without producing that condensation in mass, which would otherwise cloud the whole depth of the atmosphere with precipitating moisture. The clouds are thereby confined to parallel horizontal planes, with intermediate clear spaces, and thus arranged are offered to the influence of the sun, which dissipates their accumulations and greatly extends the expaitsive power of the elastic vapour. The power of each fluid being in proportion to its elasticity, that of the vapour compared with the air, can never, at most, exceed 1: 30: so that the general character of the mixed atmosphere is derived from the latter; which, in its irresistible motions must hurry the former along with it. The influence, however, of the vapour

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upon the air, though slower in its action, is sure in its effects, and the gradual and silent processes of evaporation and precipitation govern the boisterous power of the winds. By the irresistible force of expansion unequally applied, they give rise to updulations in the elastic fluid; the returning waves dissipate the local influence, and the accumulated effect is annihilated, again to be reproduced.

In tracing the harmonious results of such discordaut operations, it is impossible not to pause, to offer up a humble tribute of admiration of the designs of a beneficent Providence, thus imperfectly developed in a department of creation where they have been supposed to be the most obscure. By an invisible, but ever-active, agency the waters of the deep are raised into the air, whence their distribution follows, as it were by measure and Weight, in proportion to the beneficial effects which they are calculated to produce. By gradual, but almost insensible, expansions the equipoised curente of the atmosphere are disturbed, this stormy winds arise, and the waves of the sea are lifted up; and that stagnation of air and water is prevented, which would be fatal to animal existence. But the force which operates, is calculated and proportioned: the very agent which causes the disturbance bears with it its own check; and the storm, as it vents its force, is itself setting the bounds of its own fury.

The complicated and beautiful contrivances, by which the waters are collected "above the firmament," and are at the same time "divided from the waters which are below the firmament," are inferior to none of those adaptations of INFINITE

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WISDOM, which are perpetually striking the inquiring mind, in the animal and vegetable kingdoms. Had it not been for this nice adjustment of conflicting elements, the clouds and concrete vapours of the sky would have reached from the surface of the earth to the remotest heavens; and the vivifying rays of the sun would never have been able to penetrate through the dense mists of perpetual precipitation.

Nor can I here refrain from pointing out a confirmation, which incidentally arises, of the Mosiac account of the creation of that atmosphere whose wonders we have been endeavouring to unravel. The question has been asked, How is it that light is said to have been created on the first day, and day and night to have succeeded each other, when the sun has been described as not having been produced till the fourth day? The Sceptic presumptuously replies, this is a palpable contradiction, and the history which propounds it must be false. But, Moses records that God created on the first day, the earth covered with water, and did not till its second revolution upon its axis, call the firmament into existence. Now one result of the previous inquiry has been, that a sphere unequally heated and covered with water, must be enveloped in an atmosphere of steam, which would necessarily be turbid in its whole depth with precipitating moisture. The exposure of such a sphere to the orb of day would produce illumination upon it; that dispersed and equal light, which now penetrates in a cloudy day, and which indeed is "good:" but the glorious source of light could not have been visible

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from its surface. On the second day, the permanently-elastic firmament was produced, and we have seen that the natural consequences of this mixture of gaseous matter, with vapour, must have been, that the waters would begin to collect above the firmament, and divide themselves from the waters which were below the firmament. The clouds would thus be confined to definite plains of precipitation, and exposed to the influence of the winds, and still invisible sun. The gathering together of the waters on the third day, and the appearance of dry land, would present a greater heating surface, and a less surface of evaporation, and the atmosphere during this revolution would let fall its excess of condensed moisture; and upon the fourth day it would appear probable, even to our short-sighted philosophy, that the sun would be enabled to dissipate the still-remaining mists, and burst forth with splendour upon the vegetating surface*. So far, therefore, is it from being impossible that light should have appeared upon the earth before the appearance of the sun, that the present imperfect state of our knowledge; will enable us to affirm, that, if the recorded order of creation be correct, the events must have exhibited themselves in the succession

* I am indebted to Mr. Granville Perm's admirable "Estimate of the Mineral and Mosaical Geologies," for my first hint upon this subject The greater part of this Essay had been written before I perused his work; and I was pleased to find that I had unconsciously proved the necessity of that turbid state of the aqueous atmosphere, previous to the creation of the firmament, upon the probability of which he has argued in such a masterly and convincing manner.

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which is described. The argument therefore recoils with double force in favour of the inspiration of an account of natural phenomena which, in all probability, no human mind, in the state of knowledge at the time it was delivered, could have suggested; but which is found to be consistent with facts that a more advanced state of science and experience have brought to light. If, however, it were reasonable to expect that the ways of God should in all cases be justified to the knowledge, or rather the ignorance, of man, the boldest philosopher might well pause before he applied the imperfect test of a progressive philosophy to the determination of file momentos questions involved in these considerations.

I am aware that two grand principles have been passed over in the previous investigation, which cannot but have great power in modifying atmospheric changes: I allude to the agency of the electric fluid, and the influence of the moon. But their modes of operation are, at present, too obscure, and stand too much in need of experimental elucidation to allow of their being applied with the requisite precision; and to have referred to them before, would only have been unnecessarily to complicate the subject. Since the time when the mind and the nerve of a Franklin first conceived and executed the bold design of analyzing the lightnings of the heavens, but few have been found to follow in the glorious, but dangerous, path, which he has opened. The only regular series of observations I am acquainted with, are those by Mr. Read, published in the Philosophical Transactions for 1794. They exhibit much pefsever-

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ance and ingenuity, and are of a highly interesting nature, but required to have been longer continued to have enabled him to draw general conclusions. It is to be regretted that none of the great scientific establishments, either at home or abroad, have taken up this important branch of meteorology. The aids of modern science would be well applied to elucidate the still obscure relations between electricity and meteorological phenomena. From some experiments of my own upon the subject, I am inclined to believe that the elasticity of vapour is increased when electrically charged; but I have nothing decisive to offer upjon the point. Should this opinion be confirmed, it is obvious that the influence must be more rapid, partial and powerful, than that of any distributing cause which we have hitherto contemplated.

That the different phases of the moon have some connexion with changes in the atmosphere, is an opinion so universal and popular, as to be, on that account alone, entitled to attention. No observation is more general; and on no occasion, perhaps, is the almanack so frequently consulted, as in forming conjectures upon the state of the weather. The common remark, however, goes no further than that changes from wet to dry, and from dry to wet, generally happen at the changes of the moon. When to this result of universal experience we add the philosophical reasons for the existence of tides in the aërial ocean, we cannot doubt that such a connexion exists. The subject, however, is involved in much obscurity. Mr. Howard is the only one who has treated it with the consideration which it deserves.

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In his book may be found much information upon it, the result of laborious investigation. It would be foreign to my purpose to enter at large upon this interesting ground, but the previous investigation suggests one particular view of it, which it may be useful shortly to state.

The action of the moon upon the aërial columns over which it passes, may be regarded as diminishing the force of gravity. This action must be greater in proportion as the moon approaches the earth; in proportion as it coincides with the analogous action of the sun; and in proportion as its passage over the meridian comes near to the perpendicular direction. The result of this diminution of gravity must be a general decrease of density; and its effect upon the lateral currents, an acceleration of the incoming, and a decrease of the outgoing streams. The loss of weight will thus be compensated, and the excess of elasticity hence derived, will lengthen the column. The final adjustment will, therefore, be assimilated to that which arises from an equal expansion by heat. Now the effect of the atmospheric tide has hitherto been sought for, and measured upon the surface of the earth, at the base of the column; and much conjecture and disappointment have ensued from not finding the effect as great, or as regular, as had been anticipated. But, if this view of the subject be correct, the total weight of the perpendicular column would not be affected so much as that of its horizontal sections; and the amount of the lunar influence should be sought in the variations of the differences of density between some high elevation and the level of the sea. The mean of a series

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of experiments carefully conducted with this view, when the moon is upon the meridian and at the horizon, would possibly exhibit the amount of the daily tides; their weekly increase and diminution; the influence of the moon's apogee and perigee; and that of its north and south declination. It has, however, I think, been proved that the influence is still felt at the surface of the earth; and the barometer, upon an average, stands lower at new and full moon, than at the quarters. This also would naturally be expected when it is considered that the attraction of the moon is an action upon the power of gravity, and acts instantaneously in the perpendicular direction; while the compensating effects upon the lateral currents is gradual.

Is it not possible that some of the remaining discrepancies of barometrical mensurations may be traced to this influence?

Should the speculations in which I have indulged in this Essay be found worthy of consideration, they will probably suggest some new modes of conducting meteorological experiments, and some alterations in the method of arranging the results. Any useful, practical result hence arising will amply repay the labour of the undertaking.

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AN ESSAY

UPON THE CONSTRUCTION AND USES OP A NEW HYGROMETER.

"Nec non et in conviviis mensisque nostris vasa quibus esculentum additar sudorem repositoriis linqwentia diras tempestatea pronutinant."—C. Plin. Nat. Hist., Lib. xviii.

IN the year 1812, my attention was attracted by the passage above extracted from Pliny, which appeated to me, by the interpretation which I affixed to it, to point to a natural phenomenon which might be rendered subservient not only to prognosticatiotis of the weather, according to the suggestion of that accurate observer, but to some of the more refined purposes of modern science. I was, how-ever, for: some time doubtful how far the interpretation which had occurred to me could be borne out by the translation of the expression esculentum; as it was a necessary condition to this interpretation, that whatever was served up in the vasa should have been cold.

The passage is thus rendered into English in a very old translation which I consulted: "And to Conclude, and make an end of this discourse; Whensoever you see, at any feast, the dishes and platters whereon your meat is served up to the

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board, sweat or stand of a dew, and leaving that sweat which is resolved from them either upon dresser, cupboard, or table, be assured that it is a token of terrible tempests approaching."

Translation of C. Pliny, by Philemon Holland. 1601.

Upon referring to several competent judges, they confirmed my conjecture, and agreed with me in thinking, that the dew or sweat, so accurately described as forming, in particular kinds of weather, upon vessels in which food was served up, could only have arisen from depression of temperature.

This, perhaps, will therefore be consideradas one of the most curious cases upon record, in which the sagacity of the ancients anticipated an observation which has been held to be peculiarly demonstrative of the superior refinements of the present state of experimental philosophy, and may settle a disputed claim to the honour of priority of discovery amongst the existing race of natural philosophers.

However this may be, my mind was thus directed to the deposition of moisture which takes place upon certain bodies when brought into an atmosphere which is warmer than themselves; and following up the suggestion of Pliny, I readily conceived that the fact was connected with meteorological phenomena; and that experiments, founded upon it, might be devised to elucidate the relation of air to vapour. I shortly after applied myself seriously to the inquiry, and was soon satisfied of the accuracy of the conjecture.

The manner in which I proceeded at that time, was as follows: I made a mixture of two salts calculated to produce cold by their solution; I

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then arranged half a dozen drinking-glasses upon a board, each furnished with a thermometer, and poured water into one of them. I added a tea-spoonful of the freezing mixture, which invariably produced a copious dew upon the exterior of the glass. I emptied the contents of the first glass into the second, and so into the third, &c, till the liquor, gradually acquiring heat by the process, arrived at such a temperature as no longer to produce any condensation upon the vessel. This point, as marked by the thermometer, was noted, and found to vary, very considerably, in relation to the temperature of the air, according to different states of the atmosphere.

I kept a journal of the weather for several months; registering the variations of the barometer, thermometer, De Luc's hygrometer, and the temperature at which moisture was condensed, and obtained some very interesting results.

I afterwards varied my apparatus in the following manner: I procured five small hollow cylinders of brass, three inches in diameter, and four inches in height, fitted with a small cock in the bottom of each. These were very highly polished, and placed in a frame, one immediately over another, so that by turning the cork, the contents of the upper would flow into that immediately beneath it. I put the cold liquid into the top cylinder; and when steam was produced upon its surface, suffered the solution to run into the next, and so into the third, &c., till all condensation ceased; when the temperature was marked as before. I found this apparatus very sensible, the bright surface of the metal being

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visibly obscured by the slightest film of moisture. These experiments were, however, troublesome, and required much time to ensure accuracy. The results I forbear from particularly detailing, as they are superseded by the more exact observations which I have been enabled to make with the instrument which I am about to describe.

It was not till many months after I had commenced this course of inquiry, that I discovered that the mode of investigation which had been suggested to me by the observation of the Roman naturalist, was not as new as I had conceived it to be. The same principle had been applied by the Academicians del Cimento (the restorers of experimeutal philosophy, as they have been very properly called), to the purposes of hygrometry.

They took a glass vessel of a conical form, and kept it full of snow or pounded ice. This vessel was suspended in the open air with its point downwards, and the moisture which was condensed upon it, trickled down its sides, and dropped from the point of the cone. The frequency of the drops, was applied by them, as a measure of the humidity pf the atmosphere. M. le Roi also, adopting the same idea, simplified its application by putting water into a glass vessel, and gradually lowering its temperature, by means of ice, till the appearance of a slight dew upon the surface denoted the point of saturation. The temperature of this point he measured, by means of the thermometer. He judged of the humidity of the air by the greater or less degree of depression necessary to produce pre-

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cipitation. Lastly, Mr. Dalton, in his "Essay upon the force of steam or vapour from water and other liquids at different temperatures," (one of an interesting series, read before the Literary and Philosophical Society of Manchester, and published in the fifth volume of their Memoirs, which it would be difficult to match for originality and sound philosophical induction), thus describes his method of finding the force of the aqueous vapour:—

"I usually take a tall cylindrical glass jar, dry on the outside, and fill it with cold spring-water, fresh team the well; if dew be immediately formed on the outside, I pour the water out; let it stand a while to increase in heat, dry the outside of the glass well with a linen cloth, and then pour the water in again: this operation is to be continued till the dew ceases to be formed, and then the temperature of the water must be observed. Spring-water is generally about 50°, and will mostly answer the purpose the three hottest months of the year; in other masons an artificial cold mixture is required."

The discovery of want of originality damped for a time the ardour of a laborious pursuit; but I have ever since been impressed with the great utility of any contrivance which might enable an observer to mark with precision, neatness, and expedition, the constituent temperature of atmospheric vapour. Upon reading the account of the ingenious contrivanee of Dr. Wollaston, which he has termed the Cryophorus, the subject again occurred to me; and I received from that instrument the hint, which, after many trials, led to the completion of my hygrometer.

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Fig. 1, represents the instrument in its full dimensions; a and b are two thin glass balls of 1 ¼ inch diameter, connected together by a tube, having a bore about ⅛th inch. The tube is bent at right angles, over the two balls, and the arm b c contains a small thermometer d e, whose bulb, which should be of a lengthened form, descends into the ball b. This ball having been about two-thirds filled with ether, is heated over a lamp till the fluid boils, and the vapour issues from the capillary tube f, which terminates the ball a. The vapour having expelled the air from both balls, the capillary tube f is hermetrically closed by the flame of a lamp. This process is familiar to those who are accustomed to blow glass, and may be known to have succeeded after the tube has become cool, by reversing the instrument and taking one of the balls in the hand, the heat of which will drive all the ether into the other ball, and cause it to boil rapidly. The other ball a is now to be covered with a piece of muslin. The stand g h is of brass, and the transverse socket i is made to hold the glass tube in the manner of a spring, allowing it to turn and be taken out with little difficulty. A small thermometer k I is inserted into the pillar of the stand. The manner of using the instrument is this:—After having driven all the ether into the ball b by the heat of the hand, it is to be placed at an open window, or out of doors, with the ball b so situated as that the surface of the liquid may be upon a level with the eye of the observer. A little ether is then to be dropped upon the covered ball. Evaporation immediately takes place, which, producing cold upon the ball a, causes

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a rapid and continuous condensation of the ethereal vapour in the interior of the instrument. The consequent evaporation from the included ether, produces a depression of temperature in the ball b, the degree of which is measured by the thermometer d e. This action is almost instantaneous, and the thermometer begins to fall in two seconds after the ether has been dropped. A depression of 30 or 40 degrees is easily produced, and I have seen the ether boil, and the thermometer driven down below 0° of Fahrenheit's scale. The artificial cold, thus produced, causes a condensation of the atmospheric vapeur upon the ball b, which first makes its appearance in a thin ring of dew, coincident with the surface of the ether. The degree at which this takes place is to be carefully noted. A little practice may be necessary to seize the exact moment of the first deposition; but certainty is very soon acquired. It is advisable, when the instrument has been constructed with a transparent ball, to have some dark object behind it, such as a house, or a tree; as the cloud is not so readily perceived against the open horizon. The depression of temperature is first produced at the surface of the liquid, where evaporation takes place; and the currents, which immediately ensue to effect an equilibrium, are very perceptible. The bulb of the thermometer d e, is not quite immersed in the ether, that the line of greatest cold may pass through it. In very damp or windy weather the ether should be very slowly dropped upon the ball, otherwise the descent of the thermometer will be so rapid as to render it extremely difficult to be certain of the degree. In dry weather,

L

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on the contrary, the ball requires to be well wetted more than once, to produce the requisite degree of cold. If at any time there should be reason to suspect the accuracy of an observation, it may easily be corrected by observing the temperature at which the dew upon the glass again disappears: the mean of the two observations (whose errors, if any, will lie in contrary directions,) will give the true result. It is obvious that care should be taken not to permit the breath to affect the glass. With these precautions the observation is simple, expeditious, easy, and certain.

Being desirous of ascertaining whether the superior power of metals in conducting heat, together with the high polish of which they are susceptibly might not be rendered conducive to the perfection of the hygrometer, I endeavoured to modify its form in such a way as to allow of their being employed in its construction. After some unsuccessful trials I completed one, of which the subjoined is an outline.

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The balls a and b, together with their connecting tube, are made of very thin brass. To the orifice f is soldered a small piece of platinum tube, which, from its property of welding with glass, allows of the junction of a piece of glass tube, and, after the instrument has been boiled as before directed, may be hermetrically closed in the usual way. The thermometer d e is so constructed that its bulb, which is enclosed in the ball b, is rather less than the diameter of its stem, which is made proportionally thick. It is ground air-tight into a collar of brass, made for its reception on the top of the ball. The ball a is covered with muslin, and the ball b is very highly polished. The advantages which I looked for in this construction of the instrument were two: First, I expected that an unpractised observer might more readily be able to mark with precision the instant of the first precepitation of the dew. The white mist is directly seen, whereas a little experience is required to obtain an equal degree of certainty with the transparent glass. Secondly, I imagined that its sensibility might be increased by extending, at pleasure, the scale of the thermometer d e. The divisions of the thermometer included in the glass instrument are necessarily small; but those of the external thermometer may be made of any required magnitude, without rendering the bulk of the whole inconveniently great.

It was also an important object to ascertain whether any hygrometric property of the glass, or difference between it and the metal in attraction of moisture, would have any appreciable effect upon the condensing power.

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Long experience has, however, convinced me that the metallic hygrometer possesses no real superiority over the glass. The visibility of the deposition in the latter is rendered perfect by making the condensing ball of black glass, and viewing it by reflected light in the manner of a mirror; and I never could perceive any difference in the sensibility of the two instruments.

Thus much on the construction of the hygrometer: It is simple and easy. Its graduation depends upon no arbitrary or disputed determinations of wet and dry: it is liable to no deterioration from use, age, or accidental circumstances; and above all things, whenever, or by whomsoever made, it is incapable, in proper hands, of affording erroneous results. It may be more or less boiled; the vacuum may be more or less perfect; and it may, consequently, require the affiusion of a larger or smaller quantity of ether to make it act: but (provided the thermometer be correct) the observation, when obtained, cannot deceive. Its determinations are, therefore, as strictly comparable one with another, under all circumstances, as those of the barometer or the thermometer.

In describing the various uses and applications of the hygrometer, I shall commence with the most popular; its use, namely, as a weather-glass.

When consulted with a view of predicting the greater or less probability of rain, or other atmospheric changes, two things are to be principally attended to—the difference between the constituent temperature of the vapour, and the temperature of the air; and the variation of the dew-point. In general, the chance of rain, or other precipitation of

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moisture from the atmosphere, may be regarded as in inverse proportion to the difference between the two thermometers: but in making this estimate, regard must be had to the time of day at which the observation is made. In settled weather the dryness of the air increases with the diurnal heat, and diminishes with its decline; for the constituent temperature of the vapour remains nearly stationary. Consequently, a less difference at morning or evening is equivalent to a greater in the middle of the day.

But to render the observation most completely prospective, regard must be had at the same time to the movement of the dew-point. As the elasticity of the vapour increases or declines, so does the probability of the formation and continuation of. rain. An increasing difference, therefore, between the temperature of the air, and the temperature of the point of condensation, accompanied by a fall of the latter, is a sure prognostication of fine weather; while diminished heat, and a rising dew-point, in fallibly portend a rainy season. When observations shall have been made and registered for a sufficient length of time, the mean results for the different periods of the year will afford accurate standards of comparison whereby to judge of the state of the vapour; and the three years' Journal appended to this Essay, will not be without its use in this respect. In winter, when the range of the thermometer during the day is small, the indication of the weather must be taken more from the actual rise and fail of the point of condensation, than from the difference between it and the temperature of the

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air, It must be remembered that a state of saturation may exist, and precipitation even take place in the finest weather, and under a cloudless sky; but this is when the diurnal decline of the temperature of the air, near the surface of the earth, falls below an unfluctuating term of precipitation; and it is proliable, that at some period or other of the twenty-four hours, this term is always passed. The radiation of the earth, in the absence of the sun, cools the stratum of air in contact with it; and a light precipitation takes place, of so little density as totally to escape the observation of the eye. At other times it becomes visible, and assumes the appearance of mist ot fog. Under such circumstances, the hygrometer will sometimes exhibit a difierent kind of action. If it be brought from an atmosphere of a higher temperature into one of a lower degree, in which, condensed aqueous particles are floating, the mist will begin to form at a temperature several degrees higher than that of the air. The heat emanating from the ball of the instrument, dissolves the particles of water, and forms an atmosphere around it of greater elasticity than the surrounding medium; so that, when it is put in action, the point of deposition is proportionably raised. This action does not at all interfere with the determination of the real force and quantity of vapour; for, in all such cases, the full saturation of the atmospheric temperature must have place, and, consequently, the temperature of the vapour must be coincident, with that of the air.

This kind of precipitation, which may often be detected by the hygrometer, when it would other.

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wise escape notice, far from being indicative of rain, generally occurs in the most settled weather. It is analogous to the formation of dew, and is dependant upon the same cause, the radiation of the earth, which can only take place under an unclouded sky.

A sudden change in the dew-point, is generally accompanied by a change of wind: but the formet sometimes precedes the latter by a short interval; and the course of the aërial currents may be anticipated before it affects the direction of the weather-cock, or even the passage of smoke.

My own experience, and the testimony of others, assure me that the hygrometer, thus applied, is more to be depended upon than any inetrument that has yet been proposed. Even when its indication are contrary to those of the barometer, reliance may be placed upon them; but simultaneous observations of the two most usefully correct each other. The rise and fall of the mercurial column is, most probably, primarily dependant upon the state of the upper regions of the atmosphere, with regard to heat and moisture. Local chemical alterations of its density, thus partially brought about, are mechanically adjusted, and the barometer gives us notice of what is going on in inaccessible regions. A rise in the dew-point, accompanied by a fall of the barometer, is an infallible indication that the whole mass of the atmosphere is becoming embued with moisture, and a copious precipitation may be looked for. If the fall of the barometer take place at the same time that the point of precipitation is depressed, we may conclude that the expansion which occasions the

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former has arisen at some distant point, and wind, not rain, will be the result. But when the air attains the point of precipitation, with a high barometer, we may infer that it is a transitory and superficial effect, produced by local depression of temperature. Particular illustrations of these modified effects might easily be adduced in this place, but they will be more conveniently studied in the abondant observations of the annexed journal.

Thus does the Hygrometer mark with infallible precision the comparative degrees of moisture and dryness in the atmosphere, and by exhibiting them in degrees of the thermometer, refer them to a known standard of comparison, and speak in a language which every body understands. But its observations may be made applicable to a much wider field of research, and adapted to still more important objects. By means of tables, we can find with the utmost accuracy and ease the positive weight of aqueous vapour diffused through any given portion of space, and its force or elasticity as measured by the column of mercury which it is capable of supporting: we discover at once the proportion of moisture in any space to the quantity which would be required to saturate it, or what has been termed the true natural scale of the hygrometer: we can calculate with perfect ease the specific gravity of any mixture of air and aqueous vapour: and we can measure the force and quantity of evaporation. Upon the data employed in the construction of the tables, it will be necessary that I should premise a few reflections.

Mr. Dalton, in his valuable Essay before referred to, has detailed the results of a laborious series of

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experiments, by which he has ascertained with great precision the force of vapour from water at every degree between its freezing and its boiling points, and derived from them a formula, by which he extended the results from the freezing of mercury to the 325th degree of Fahrenheit's scale. Dr, Ure has since* entered upon the same investigation with à different modification of apparatus, calculated to avoid some irregularities to which Mr. Dalton's was exposed. He carried his actual experiments as high as 312°; and thus ascertained that Mr. Dalton's ratio of progression for the force of vapour, though apparently accommodated to the intervals between 32° and 212°, could hot serve for the higher ranges. In the prosecution of the inquiry, he was led to the discovery of .a very simple ratio, which admirably connected together the whole series of experiments. In a former essay upon this subject, I adopted Mr. Dalton's numbers; which, for the range of atmospheric temperature, exhibited not only a perfect adaptation to his own experiments, but also a surprising accordance with those of Dr. Ure: but reflecting that from these and other considerations, the rule from which they were derived could not be the law of nature, I have recalculated the tables from the data of Dr. Ure. It 'is gratifying to find that, for the purposes of the hygrometer, the difference after all is very inconsiderable.

The second column of Table I. exhibits the force of aqueous vapour, hence derived, in inches of mer-

* Phil. Trans. 1818., p. 338.

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cury, at the temperature marked in the corresponding line of the first column.

Upon these two data, namely, the force and temperature of the vapour, are chiefly founded the calculations which have furnished me with the series of the third column, which, contains, the weight in grains of a cubic foot of the vapour at the corresponding temperature and pressure. The method of computing it is as follows:—Steam at 212°, and under a pressure of 30 indies of mercury, is, as nearly as possible, 1700 times lighter than an equal bulk of water at its maximum of density; and a cubic foot of water, at the temperature of 40° weighs, according to the accurate investigations of Dr. Rice, 437373 grains; the weight, therefore, of a cubic foot of steam, at the above temperature and pressure is 4 3 7 2 7 2/1 7 0 0, or 257.218 grains. Hence we may find the weight of an equal bulk of vapour of the same temperature under any other given pressure, suppose 0.560 in.: for the volume being in inverse proportion to the pressure,

Ins. Ins. Grs. Grs.
30 : 0.560 :: 257.218 : 4.801

the weight required.

Having now obtained the weight of a cubic foot of vapour, at a pressure of 0.560 in., and at a temperature of 212°, we may proceed to find its weight under the same pressure at any other temperature, suppose 60°. It has been fully established, that all aëriform bodies (vapours out of the contact of their respective fluids, as well as gases,) expand 1/480th part of their volume for every accession of temperature, equivalent to one degree of Fahrenheit's

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scale; therefore, reckoning a volume of gas at 32° as unity, its volume at 60° is to its volume at 212°, as 1 + 28/480 is to 1 + 180/480, or:: 1.0583: 1.3749, and the density and weight being in inverse proportion to the volume,

vol. at as. vol. at 212. Grs. Grs.
1.0583 : 1.3749 :: 4.801 : 6.222

the weight of the cubic foot of vapour at the temperature of 60° and under a pressure of .560 in.

It has also been proved by Mr. Dalton, that as much vapour of determined temperature is formed in a given bulk of air as in a vacuum of equal space; therefore, the above result gives the weight of vapour which can exist in a cubit foot of air at the temperature of 60°. The fourth column of the table contains the proportionate expansion for the corresponding degrees.

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TABLE I. Shewing the Force, Weight, and Expansion of Aqueous Vapour, at different Temperatures, from 0° to 95°

Temp. Forcs. Weight of a Cabic Foot. Expanasion.
0 0.068 0.856 .9334
1 0.071 0.892 .9355
2 0.074 0.998 .9375
3 0.077 0.963 .9396
4 0.080 0.999 .9417
5 0.083 1.034 .9438
6 0.086 1.069 .9459
7 0.089 1.104 .9480
8 0.092 1.139 .9500
9 0.095 1.173 .9521
10 0.098 1.808 .9542
11 0.103 1.254 .9563
12 0.107 1.308 .9584
13 0.111 1.359 .9605
14 0.115 1.405 .9625
15 0.119 1.451 .9646
16 0.123 1.497 .9667
17 0.127 1.541 .9688
18 0.131 1.586 .9709
19 0.135 1.631 .9730
20 0.140 1.688 .9750
21 0.146 1.757 .9771
22 0.152 1.825 .9792
23 0.158 1.893 .9813
24 0.164 1.961 .9834
25 0.170 8.028 .9855
26 0.176 2.096 .9875
27 0.182 2.163 .9896
28 0.188 2.229 .9917
29 0.194 2.295 .9938
30 0.200 2.361 .9959
31 0.208 2.451 .9980
32 0.216 2.539 1.0000
33 0.224 2.630 1.0020
34 0.232 2.717 1.0041
35 0.240 2.805 1.0062
36 0.248 8.892 1.0083
37 0.258 2.979 1.0104
38 0.264 3.066 1.0125
39 0.272 3.153 1.0145
40 0.280 3.239 1.0166
41 0.292 3.371 1.0187
42 0.304 3.502 1.0208
43 0.316 3.633 1.0229
44 0.328 3.763 1.0250
45 0.340 3.893 1.0270
46 0.352 4.022 1.0291
47 0.364 4.151 1.0312
48 0.376 4.279 1.0333
49 0.388 4.407 1.0354
50 0.400 4.535 1.0375
51 0.414 4.684 1.0395

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Temp. Forcs. Weight of a Cabic Foot. Expanasion.
52 0.428 4.832 1.0416
53 0.444 5.003 1.0437
54 0.460 5.173 1.0458
55 0.476 5.342 1.0458
56 0.492 5.511 1.0500
57 0.508 5.679 1.0520
58 0.526 5.868 1.0541
59 0.543 6.046 1.0562
60 0.560 6.222 1.0583
61 0.577 6.399 1.0604
62 0.594 6.575 1.0625
63 0.615 6.794 1.0645
64 0.636 7.013 1.0666
65 0.657 7.230 1.0687
66 0.678 7.447 1.0708
67 0.699 7.662 1.0729
68 0.722 7.899 1.0750
69 0.745 8.135 1.0770
70 0.770 8.392 1.0791
71 0.796 8.658 1.0812
72 0.822 8.924 1.0833
73 0.849 9.199 1.0854
74 0.877 9.484 1.0875
75 0.906 9.780 1.0895
76 0.936 10.107 1.0916
77 0.966 10.887 1.0937
78 0.997 10.699 1.0958
79 1.028 11.016 1.0979
80 1.060 11.833 1.1000
81 1.093 11.664 1.1000
82 1.127 12.005 1.1041
83 1.162 12.354 1.1062
84 1.198 12.713 1.1083
85 1.235 13.081 1.1104
86 1.273 13.458 1.1125
87 1.312 13.877 1.1145
88 1.351 14.230 1.1166
89 1.390 14.613 1.1187
90 1.430 15.005 1.1208
91 1.470 15.432 1.1229
92 1.510 15.786 1.1250
93 1.551 16.186 1.1270
94 1.593 16.593 1.1291
95 1.636 17.009 1.1312
212 30.000 257.218 1.3749

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It is at all times desirable to bring the results of calculation, however exact the data, upon which they are founded, to the test of actual experience; and wp have the ready means of so doing with regard to the above table. The indefatigable Da Saussure, in his "Essais sur L'Hygrométrie," gives the results of a series of experiments, to determine the quantity of moisture which air is capable of dissolving at certain temperatures. The means which he employed were simple. He thoroughly dried the air of a large glass balloon, of known capacity; and then suspended in it a small piece of linen, which had been moistened and accurately weighed. He ascertained the point of saturation by means of a manometer, which ceased to move when the term of extreme humidity had been obtained, and then withdrawing the linen, he instantly noted its loss of weight. He thus found that at the temperature of 15°. 16 Reau. a French cubic foot of air took up 11.0690 grains of water; while at 6°.18 Reau. it only dissolved 5.6549 grains. By reducing these results to English weights and measures, we have at 66° of Fahrenheit, 7,498 grains in a cubic foot, and at 45½° Fahrenheit, 3,830 grains: a wonderful accordance with our theoretical determinations.

Mr. Anderson, in his highly interesting treatise upon Hygrometry, published in the "Edinburgh Encyclopædia," has also given us the results of his experiments, to determine the same point by a method less liable, perhaps, to objection. His manner of operating consisted in causing a large volume of air, saturated with moisture, to pass slowly in a stream through a sufficient quantity of sulphuric

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acid, or dry muriate of lime, cut off from all communication with the atmosphere; and then observing the increase of weight which these substances ac quired in consequence of the air being transmitted through them. The weight of a cubic foot of steam, at different temperatures hence derived, is compared in the following table with those derived from calculation.

Temp. Grs. by Expt. Cakalafcd.
49 4.085 4.407
69 5.679 6.046
77 9.828 10.387
83 11.660 12.354

Considering the nature of the experiment, and the complication of the calculations, this is again a very close agreement.

The manner of using the table will, perhaps, be best understood from an example. Let the temperature of the atmosphere be 70°, and the point of condensation, as found by the hygrometer, 55°; the pressure of the vapour, under these circumstances, is immediately found opposite to the degree of its constituent heat 55° = 0.476. To find its weight, we proceed thus:—Supposing that the temperature of the air had not differed from that of the dew-point, its weight would have been found upon the same line as its pressure = 5.342 grains. But its bulk is expanded by the excess of atmospheric heat; we must, therefore, seek in the fourth column for the degree of expansion at 55°= 1.0479, and at 70° = 1.0791, and apply the correction thus:—

Bulk at 70° Bulk at 55° Grs. Grs.
1.0791 : 1.0579 :: 5.342 : 5,175

which is the weight required.

[page] 161

Again,—the dryness of the atmosphere, under the above conditions, may be conveniently expressed as 15°, in terms of the thermometric scale: but it may be desirable also to know what it would be upon the natural scale of the hygrometer. This is readily ascertained by dividing the elasticity of vapour at the temperature of the dew-point, by the elasticity at the temperature of the air: the quotient will express the proportion of moisture actually existing, to the quantity which would be required for saturation; for, calling the term of saturation 1.000, as the elasticity of vapour at the temperature of the air is to the elasticity of vapour at the temperature of the dew-point, so is the term of saturation to the actual degree of moisture,—thus,

Elas. at 55. Blast, at 70.

.476 ÷ .770 = .618

The relation of this mode of expressing the degree of moisture to that of denoting the degree of dryness, by the thermometric scale, will be elucidated by selecting a different example. Let the temperature of the air be 47°, and the dew-point 32°; the dryness represented by the former expression will be 15°, as before, but by the latter the degree of moisture will be reduced to .593.

Thus, by two simple observations, and very easy calculations, we ascertain, with precision, the following points of the utmost interest to meteorology.

Temperature of the air 70°
Dew-point 55°
Degree of dryness on the thermometric scale 15°
Degree of moisture on the hygrometric scale 618
Elasticity of the vapour 476 ins.
Weight of vapour in a cubic foot

M

[page] 162

The state of the atmosphere, assumed above, would constitute fine weather; and one of two things, or a modification of both, must happen, before any precipitation of water could take place: either the temperature of the air must fall below 55°; or the quantity of vapour must increase to 8.392 grains in the cubic foot, the maximum quantity which could exist at 70°; or the point of condensation may become intermediate, by a corresponding rise and fall of the two.

In the first case, the precipitation would probably be only slight and transitory, such as mist or fog: in the second case, it would assume the form of hard rain and storms: while, in the third, some conjecture might be formed of its probable duration and quantity, according as one or other of its causes prevailed.

But the hygrometer can be made to measure not only the quantity and force of vapour existing at any time in the air, but may be applied at the same time to indicate the force and quantity of evaporation. Mr. Dalton, in the course of that important train of investigation to which I have before had occasion to refer, ascertained that the quantity of water, evaporated in a given time, bore an exact proportion to the force of vapour at the same temperature. The atmosphere obstructs its diffusion, which would otherwise be instantaneous, as in vacuo; but this obstruction is overcome with a celerity proportioned to the force of the vapour. The retardation, however, does not arise from the weight of the air, for that would prevent any vapour from rising under 212°; but, as Mr. Dalton observes, is caused by the viz inertiœ of the particles of air, and is simi-

[page] 163

lar to that which a stream of water meets with in descending amongst pebbles. In ascertaining this point at ordinary atmospheric temperatures, regard must be had to the force of vapour already existing in the air. For instance, if water of 57° were the subject, the force of vapour of that temperature is 1/60 th of the force at 212°; and one might expect the quantity of evaporation to be 1/60 th also; but if it should happen that an aqueous atmosphere to that amount does already exist, the evaporation instead of being 1/60 th of that from boiling water, would be nothing at all. On the other hand, if the aqueous atmosphere were less than that, suppose half of it, then the effective evaporating force would be 1/120 th of that from boiling water; in short, the evaporating force must be universally equal to that of the temperature of the water diminished by that already existing in the atmosphere. But the air, by its mechanical action has another influence upon the rate of evaporation. When calm and still, it merely obstructs the process; but when in motion, it increases its effect in direct proportion to its velocity, by removing the vapour as it forms. Mr. Dalton fixes the extremes that are likely to occur in ordinary circumstances at 120 and 189 grains per minute, from a vessel of six inches diameter, at a temperature of 2l2°. Upon these data the following Table was constructed, in which Mr. Dalton's results have been accommodated to the progression of elasticity adopted in Table I., from Dr. Ure, by the slight alteration of moving the temperature back two degrees.

M 2

[page] 164

TABLE II. Shewing the Force of Vapour, and the full evaporating Force of every Degree of Temperature, from 18° to 85°; expressed in Grains of Water that would be raised per Minute from a Vessel of Six Inches in Diameter, supposing there were no Vapour already in ike Atmosphere.

Temp. Force of Vapour. Evap. Force in grs.
212° 30.000 120 gr. 154 gr. 189 gr.
18 .131 0.52 0.67 0.82
19 .135 0.54 0.69 0.85
20 .140 0.56 0.71 0.88
21 .146 0.58 0.73 0.91
22 .152 0.60 0.77 0.94
23 .158 0.62 0.70 0.97
24 .164 0.65 0.82 1.02
25 .170 0.67 0.86 1.05
26 .176 0.70 0.90 1.10
27 .182 0.72 0.93 1.13
28 .188 0.74 0.95 1.17
29 .194 0.77 0.99 1.21
30 .200 0.80 1 03 1.26
31 .208 0.83 1.07 1.30
32 .216 0.86 1.11 1.35
33 .224 0.90 1.14 1.39
34 .232 0.92 1.18 1.45
35 .240 0.95 1.22 1.49
36 .248 0.98 1.26 1.54
37 .256 1.02 1.31 1.60
38 .264 1.05 1.35 1.65
39 .272 1.09 1.40 1.71
40 .280 1.13 1.45 1.78
41 .292 1.18 1.51 1.85
42 .304 1.22 1.57 1.92
43 .316 1.26 1.62 1.99
44 .328 1.31 1.68 2.06
45 .340 1.36 1.75 2.13
46 .352 1.40 1.80 2.20
47 .364 1.45 1.86 2.28
48 .376 1.50 1.92 2.36
49 .388 1.55 1.99 2.44
50 .400 1.60 2.06 2.51
51 .414 1.66 2.13 2.16
52 .428 1.71 2.20 2.69
53 .444 1.77 2.28 2.78
54 .460 1.83 2.35 2.88
55 .476 1.90 2.43 2.98
56 .492 1.96 2.52 3.08
57 .508 2.03 2.61 3.19
58 .526 2.10 2.70 3.30
59 .543 2.17 2.79 3.41
60 .560 2.24 2.88 3.52
61 .577 2.31 2.98 3.63
62 .594 2.39 3.07 3.76
63 .615 2.46 3.16 3.37
64 .636 2.54 3.27 3.99
65 .657 2.62 3.37 4.12
66 .678 2.70 3.47 4.24
67 .699 2.79 3.59 4.38
68 .722 2.88 3.70 4.53
69 .745 2.98 3.83 4.68
70 .770 3.08 3.96 4.84
71 .796 3.18 4.09 5.00
72 .822 3.29 4.23 5.17
73 .849 3.40 4.37 5.34
74 .877 3.52 4.52 5.53
75 .906 3.65 4.68 5.72
76 .936 3.76 4.83 5.91
77 .966 3.88 4.99 6.10
78 .997 4.00 5.14 6.29
79 1.028 4.16 5.35 6.54
80 1.060 4.28 5.50 6.73
81 1.093 4.40 5.66 6.91
82 1.127 4.56 5.86 7.17
83 1.162 4.68 6.07 7.46
84 1.198 4.80 6.28 7.75
85 1.235 4.92 6.49 8.04

[page] 165

The first column contains the degrees of temperature; the second the corresponding force of vapour; the third the amount of evaporation, per minute from a vessel of six inches' diameter in calm weather; the fourth, the amount in a moderate. breeze; and the fifth, in a high wind.

The use of this table, as applied to the hygrometer, is this:—Let it be required to know the force of evaporation at the existing state of the atmosphere: find the point of condensation by the instrument, as before directed; subtract the grains opposite that temperature, either in the third, fourth, or fifth columns, according to the state of the wind, from the grains opposite to the temperature of the air in the same column, and the remainder will be the quantity evaporated in a minute from a vessel of six inches' diameter, under the given circumstances. For example;—Let the point of condensation be 55°, the temperature of the air 70°, with a moderate breeze. The number opposite to 55° in the fourth column is 2.43, and that opposite to 70° is 3.96: the difference, 1.53 grain, is the evaporation per minute.

But it is, perhaps, simpler and more convenient, in many cases, to estimate the depth of the water evaporated in a day, and Dr. Young has shewn how this may be done, from Mr. Dalton's data. It happens that the column of mercury equivalent to the elasticity of the vapour, expresses, accurately enough, the mean evaporation in 24 hours. Mr. Dalton's experiment gives 45 grains per minute, from a disc of 3¼ inches. Now 45 × 60 × 24 = 64800 grains, or 256.6 cubic inches, which would

[page] 166

make a cylinder 30.9 inches in height, on a base 3¼ inches in diameter; and this differs only 1/33 from the height of the column of mercury. We may, therefore, assume that the mean daily evaporation is equal to the tabular number expressing the elasticity of the vapour; sometimes exceeding it, or falling short of it about one-fourth; and we may readily allow for the effect of the moisture of the atmosphere, by deducting the number corresponding to the temperature of deposition. Thus, supposing the mean temperature, of 24 hours to be 60°, and that of the dew-point 50, the evaporation will be equal to .560 — .400 =,160 inch.

It is evident that these estimates can be but more approximations; for till we can obtain some accurate measure of the velocity of the wind, they must be liable to great uncertainty. They are, however, as much to be relied upon as the registers of the eyaporating gauge in common use, whose only proper application can be to furnish a rough estimate of the state of atmospheric saturation, and the poipt of deposition. The notion that they afford the absolute measure of the quantity of water raised into the air is absurd, for the instrument can only give the amount of evaporation from the shallow body of water in the place where it has been fixed. The conditions which modify the process vary almost ad infinitum. They vary on the land and on the water; they vary in sun-shine and in the shade;. they vary as land is more or less clothed with vegetation, or as water is more or less deep. The evaporating gauge, so far from representing the circumstance of those bodies which yield the great

[page] 167

body of vapour on the earth's surface, probably does not correspond, in all essential particulars, with a dozen puddles in the course of the year; and the pains which are often taken to make the results tally with those of the rain-gauge, or to compare the two, are wholly misdirected. The results of the hygrometer, as applied above, accommodate themselves more easily to the ever-varying conditions of the problem; and from these we can infer the effect of each combination of circumstances, and the capacity of the air for moisture modified by the velocity of the winds.

The next application of the hygrometer is not of inferior importance to any of those which we have been considering: I allude to its application to the correction of barometrical measurements. Ever since the celebrated and important experiment of Torricelli, the attention of some of the greatest philosophers has been drawn in succession to the interesting problem of the mensuration of heights by means of the barometer. The most laborious experiments have been undertaken for the improvement of the practical part of the operation, and the utmost refinements of mathematical calculation have been employed in the perfecting of its theory. To the former, M. de Luc, General Roy, and Sir George Shuckburgh, have pre-eminently contributed; while the powerful minds of Hailey, Newton, Playfair, and Laplace, have been applied to the latter. But one desideratum in Physics has stopped the progress of each at nearly the same point; a desideratum which all have felt, and all in succession have pointed out. I allude to the deficiency of means to

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measure the quantity and effects of aqueous vapour in the atmosphere. The relation of the air's density and elasticity, the effects of heat upon the relative weights of mercury and air, the diminution of gravity in ascending from the surface of the earth, its variation in different latitudes, and the disturbance of centrifugal force, have been appreciated and allowed for; but all the corrections, excepting the two first, are exceeded in value by that which has hitherto been only the subject of conjecture; namely, the correction for moisture. Some of the latter calculations have, indeed, assumed an appearance of considerable accuracy; but while the more important problem remains unsolved, such appearance is only illusory; and it may even be doubted whether the state of physical science is ever likely to be such as to render the introduction of the refinements which they exhibit, practically advantageous. The importance, however, of the problem, the solution of which I am now about to attempt, has, on the contrary, been universally admitted.

M. de Luc, in his valuable and laborious "Researches upon the Modifications of the Atmosphere," thus adverts to the knowledge which it is necessary to obtain of the effects of vapour in the air for the perfecting the mensuration of heights by means of the barometer.

"Voilà donc un nouveau champ ouvert aux expériences. Il s'agit de détérminer quel changement on doit faire à la hauteur trouvée par les logarithmes, quand l'air est plus ou moins chargé de vapeurs qu'un certain point fixe et de vapeurs

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échauffées plus ou moins qu'un certain dégré. Il me semble que pour découverir cette loi, il faudroit pouvoir joindre l'observation d'un hygromètre comparable à celle du baromètre et thermomètre car le point essentiel consiste à connoître s'il y a des vapeurs dans la colonne d'air qui est interceptée par les deux stations et quelle est leur quantité; puisque, si les vapeurs qui font baisser le baromètre sont plus élevées que cette colonne, elles ne changent point la loi générale qui sert de fondement au calcul.

Lorsqu'on aura obtenu ce premier point, il sera focile de connoître par l'expérience. 1°, Si les vapeurs influent de la méme manière quelque soit la densité de l'air produite par la pression supérieure, et par consequent, quelque soit la hauteur du mercure dans la baromètre. 2°, Quel rapport il y a entre la quantité des vapeurs exprimée par les degrés de l'hygromètre, et la diminution d'élasticité del'air par une température donnée; ou plus directement, quelle partie proportioned il faut déduire de la hauteur trouvé par le calcul, où ajouter à cette hauteur, pour chaque degré de l'hygromètre quand Pair est à cette température. 3°, Enfin, quelle modification doit éprouver ce rapport lorsque la chaleur est plus où moins grande que le point fixe, auquel la force expansive des vapeurs est égale à celle de l'air.

Je conviens que tout cela présente bien des soins et des peines au premier coup-d'œil, mais j'ai éprouvé plus d'une fois que les difficultés connues s'applanissent beaucoup quand on les affronte avec courage."—Tome iii., p. 288.

[page] 170

General Roy, in commenting upon his experiments upon the different expansions of dry and moist air, for the elucidation of the same subject, says:—

I am aware it will be alleged, that the proportion of moisture admitted into the manometer in these experiments, is greater than what can ever take place in nature; and, therefore, in order to be able to judge of the degrees of expansion the medium suffers in its more or less dense, and more or less moist, states, that not only air near the surface of the earths but likewise that found at the top of some very high mountains should have been made use of. I grant all this; but, on the other hand, it must be remembered, that those experiments are very recently finished; that a good hygrometer, (if such can ever be obtained), a great deal of leisure time, and the vicinity of high mountains, were all necessary for the carrying of such a scheme into execution. It is for these reasons, and in hopes that other people will sooner or later investigate this matter still further, not only by experiments made on the expansion of air taken at different heights above the level of the sea in middle latitudes, but likewise on that appertaining to the humid and dry regions of the atmosphere towards the equator and poles, that I have been induced to hasten the communication of this paper. In the mean time, having proved beyond the possibility of a doubt, that a wonderful, difference doth exist between the elastic force of dry and moist air, I may be allowed hereafter to reason by analogy on the probable effects this will produce in measuring

[page] 171

heights by the barometer,"—Phil. 'Trans. vol. lxvii., p. 714;

M. la Place, who has applied the prodigious powers of his science to the perfecting the barometric formula, and has availed himself of all the accuracy of the modern method of experiment, was forced to leave the hygrometric state of the air in the catalogue of inevitable errors, contenting him-self with an approximate correction:—

"Les corrections," says he, "relative à la latitude, et à la variation de la pesanteur, sont trèspetites, mais commes elles sont certaines il est utile deles employer pour ne laisser subsister dans le calcul que les erreurs inévitables des observations, et celles qui résultent des attractions inconnues des montagnes, de l'état hygrométrique de l'air, auquel il serait nécessaire d'avoir égard et enfin de la hypothèse adoptée sur la loi de la diminution de la chaleur. On tiendrait compte en partie, de l'état hygrométrique de l'air en augmentant uh peu le coef ficient 0.00375 de t + l/2 dans la formule précédente; car La. vapeur aqueuse est plus légère que l'air, et l'accroissement du témperature en accroit la quantité toutes choses égales d'ailleurs,"—Mécanique Céleste, Tom, iv., p. 292.

The late lamented Mr. Playfair, in an elaborate paper upon the same subject, published in the Philosophical Transactions of Edinburgh, (vol i. 1778,) thus enforces the same argument: "There is another cause of error, which, had the effects of it been sufficiently known, ought, no doubt, to have entered into this investigation. Moisture, when

[page] 172

chemically united to air, or dissolved in it, so as to form part of the same homogeneous and invisible fluid, appears to have a powerful effect to increase the elasticity of the air and its expansion, for every additional degree of heat which it receives. Though the judicious and accurate experiments of General Roy have ascertained this effect of humidity, and have even gone far to determine the law of its operation; yet, for want of a measure of the quantity of it contained at any given time in the air, it is impossible to make any application of this knowledge to the object under our consideration."

Lastly, Mr. Leslie, in an article upon barometrical measurements in the Supplement to the Encyclopedia Britannica, concludes his detail of corsections with the same acknowledgment. "The humidity of the air also materially affects its elasticity, and the hygrometer should therefore be conjoined with the thermometer in correcting baromertrical observations. But nothing satisfactory has yet been done with regard to that subject. The ordinary hygrometers, or rather hygroscopes, are mere toys, and their application to science is altogether hypothetical."

Impressed with the importance of the object, so clearly pointed out by a succession of the most able philosophers, I had no sooner succeeded in constructing an instrument which, upon unerring principles, would shew the quantity of vapour contained at any given time in the air, than I turned my attention to render it available to the desired purpose; and I shall now endeavour to explain a method of observation and calculation which, I

[page] 173

trust, will be found fully and strictly to solve this important problem.

The most simple way of considering the subject, in a general point of view, appears to me to be that which was, I believe, first suggested by Sir George Shuckburgh (Phil. Trans, vol. lxvii. p. 556.), namely, to make a comparison of the specific gravities of mercury and air at a fixed temperature, and under a given pressure, the foundation of the operation. In this manner we calculate the height of a column of air, compared with any given column of mercury of equal base, supposing it of equal density throughout. The calculation of the gradual diminution of density which takes place for equal ascents in the atmosphere, according to a geometrical progression, is made in the usual way, by means of logarithms. This latter calculation may be deemed invariable under all circumstances; the former includes all the adventitious circumstances, and all the effects of disturbing causes.

The well-known accuracy of M.M. Biot and Arago, assisted by the nicety of modern instruments, has determined the relative specific gravities of dry air and mercury at a temperature of 32°, and under a pressure of 30.00 inches, to be as 1 to 10435. The height of a column of air, therefore, of equal density throughout, which would balance a column of mercury of 30 inches, under these conditions, would be very nearly 26090 feet.

Now, these proportions may be disturbed in two ways by the operation of heat. In the first place, its expansive power, acting upon the mercury, may dilate or contract its particles; so that a column of 30 inches, being more or less dense, will require an

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equipoise of greater or less length, according as its temperature is below or above the standard at 32°. This effect has been most minutely appreciated, and its correction is applied with the utmost ease and precision. In the second place, the power of heat, acting upon the air, occasions a much more considerable dilatation or contraction of its parts, and gives rise to much greater differences in the height of the equiponderant column. The expansion of air has been determined with precision by the experiments of M. Gay-Lussac, and from them we infer, with confidence, that it increases or diminishes 1/480 th part for every addition or subtraction 1° of heat. In this situation, therefore, the operation stands: the column of mercury, which is the measure applied, is rendered an invariable standard of comparison, by being brought by an easy calculation to a known density; and the altitude measured is in proportion to the specific gravity of the air.

But heat is not the only agent which alters the specific gravity of the air; the admixture of aqueous vapour, it is well known, produces very important changes in its density. It did not, as I have shewn, escape the observation of General Roy, that air, in contact with water, expanded much more than dry air; and, from well-conducted experiments, he ascertained that the expansion was greater for equal increments as the temperature rose. From the mean results which he obtained, the following increasing rates of expansion were derived:—1000 parts of air, in contact with water, and under a pressure of 32.18 inches, expanded for each degree

From 0 to 32 2.22799.
32 52 2.58800

[page] 175

From 52 78 2.97228
72 92 3.63194
92 112 4.91072
112 132 6.86550
132 152 9.89494
152 172 12.04087
172 192 17.88344
192 212 19.22470

Hence we have the progressive scale of expansion as follows:—

0 100000.
32 107129
52 112305
72 118250
92 125514
112 135375
132 149106
152 168890
172 192978
192 228744
212 267194

Modern experiments, upon the elasticity of vapour, afford us the data for calculating a similar table. A volume of dry air, of given elasticity, being mixed with vapour, also of known elasticity, will have its volume increased in proportion to the elasticity of the mixture. Thus, a cubic foot of air, of the temperature of 212°, which would support a column of mercury of 30 indies, being mixed with a cubic foot of vapour of 212°, also of the. elasticity of 30 indies, would occupy a space of two cubic feet

For 30 inches: 60 inches:: 1: 2.

We are acquainted with the force of vapour for every degree of a very long range of the thermometric scale, and if we assume the volume of dry air, at 0° temperature and 30 inches' pressure, as

[page] 176

1.00000, the calculation is sufficiently simple. For example: let it be required to know the expansion which would take place in air, in contact with water, by a rise from 0° to 32°. The force of vapour of that temperature, according to Dr. Ure's table, is 0.216 indies; therefore

30.000: 30.216:: 1.00000: 1.00720

This is the expansion which would arise from vapour only: to this we must add the expansion which would take place from the addition of heat. Now .00223 (the expansion per degree for the bulk at 0°,) × 32 = .07802; which, added to 1.00720, makes the total expansion 1.08522,

In this manner I have calculated the following scale, corresponding with that of General Roy—

0 1.00000
32 1.08528
52 1.13021
72 1.18786
92 1.25521
112 1.34011
132 1.45056
152 1.60016
172 1.80666
192 2.09151
212 2.47276

This comparison exhibits a very surprising degree of accordance; and, considering the disadvantages with which the General had to struggle, does infinite honour to his accuracy and perseverance. Little more was wanting to have enabled him to complete the solution of the problem towards which he contributed so many steps, than an instrument to measure the quantity of the agent, the eflfects of which he so well appreciated.

[page] 177

But the expansion which vapour causes in air, is not precisely similar to that occasioned by heat; for while it dilates its parts, it adds its own weight to the mixture. Let it be required to know the specific gravity of air at 32°, saturated with vapour, compared with dry air, at the same temperature. Call the latter 1.00000: the quantity of expansion will be .00720; which, deducted from 1.00000, leaves .99280. Now the weight of a cubic foot of air, under the conditions above named, is 558.131 grains, and the weight of a cubic foot of vapour at 32°, is 2.539 grains; which, the former being 1.00000, will be nearly .00455; and which, added to the .99280 before obtained, will give .99735 for the specific gravity sought.

Upon this principle I have constructed the following table, by means of which the specific gravity of any mixture of atmospheric air and aqueous vapour from 0° to 90°, may readily be found with sufficient precision. I have made air, under a pressure of 30 indies of mercury and at the temperature of 32°, the standard of comparison. The first column contains the degrees of Fahrenheit's thermometer; the second shews the quantity due to each degree of heat, to be subtracted, or added, according as the temperature is above or below the standard; the third exhibits the expansion of volume occasioned by vapour of the respective degrees of elasticity appropriate to the several degrees of heat, and is always to be subtracted; the fourth is the correction to be applied for the weight of the vapour, and is constantly to be added; and the fifth is the correct specific gravity, supposing the air saturated with moisture at the given temperature.

N

[page] 178

TABLE III. For finding the Specific Gravity of any Mixture of Air and Aqueous Vapour, fromto 90°.—Dry Air at 32 Temp and 30° Inches' Pressure, being 1.00000.

Temp. Alteration of Volume from Heat. Alteration of Volume from Vapour. Increase of Density from Weight. Correct Spceific Gravity of satuated Air.
0 +.06666 -.00226 +.00153 1.06593
1 +.06458 -.00237 +.00159 1.06381
2 +.06249 -.00247 +.00166 1.06168
3 +.06041 -.00257 +.00172 1.05956
4 +.05833 -.00267 +.00179 +.05745
5 +.05624 -.00277 +.00185 1.05582
6 +.05416 -.00287 +.00191 1.05320
7 +.05208 -.00297 +.00197 1.05108
8 +.04999 -.00307 +.00204 1.04898
9 +.04791 -.00317 +.00210 1.04684
10 +.04583 -.00343 +.00224 1.04255
11 +.04374 -.00343 +.00224 1.04255
12 +.04166 -.00357 +.00234 1.4043
13 +.03958 -.00370 +.00243 1.03831
14 +.03749 -.00384 +.00251 1.03616
15 +.03541 -.00397 +.00260 1.03404
16 +.03333 -.00410 +.00268 1.03191
17 +.03124 -.00423 +.00276 10.2977
18 +.02916 -.00437 +.00284 1.02763
19 +.02708 -.00437 +.00292 1.02550
20 +.02475 -.00467 +.00302 1.02310
21 +.02291 -.00487 +.00314 1.02118
22 +.02083 -.00507 +.00327 1.01903
23 +.01874 -.00527 +.00339 1.01686
24 +.01666 -.00547 +.00351 1.01470
25 +.01458 -.00567 +.00363 1.01254
26 +.01249 -.00587 +.00375 1.01037
27 +.01041 -.00607 +.00387 1.00821

[page] 179

Temp. Alteration of Volume from Heat. Alteration of Volume from Vapour. Increase of Density from Weight. Correct Spceific Gravity of satuated Air.
28 +. -.27 +. 1.00
29 +.00624 -.00647 +.00411 1.00
30 +.00416 -.00667 +.00423 1.00172
31 +.00208 -.00694 +.00439 0.99953
32 .00000 -.00717 +.00454 0.99737
33 -.00208 -.00747 +.00471 0.99516
34 -.00416 -.00773 +.00486 0.99297
35 -.00624 -.00800 +.00502 0.99078
36 -.00833 -.00827 +.00518 0.98858
37 -.01041 -.00854 +.00533 0.98638
38 -.01249 -.00880 +.00549 0.98420
39 -.01248 -.00907 +.00564 0.97199
40 -0.1666 -.00934 +.00580 0.97980
41 -.01874 -.00974 +.00604 0.97756
42 -.02083 -.01014 +.00627 0.97530
43 -.02291 -.01054 +.00650 0.97305
44 -.02475 -.01094 +.00674 0.97405
45 -.02708 -.01134 +.00697 0.95855
46 -.02016 -.01174 +.00720 0.96630
47 -.03124 -.01214 +.00720 0.96405
48 -.03833 -.01254 +.00766 0.96179
49 -.03541 -.01294 +.00789 0.95954
50 -.30749 -.01334 +.00803 0.95720
51 -.03958 -.01426 +.00839 0.95501
52 -.04166 -.01426 +.00864 0.95272
53 -.04374 -.01480 +.00896 0.95042
54 -.04583 -.01584 +.09926 0.94809
55 -.04791 -.01586 +.09957 0.94809
56 -.04999 -.01640 +.00987 0.94848
57 -.05208 -0.1694 +.01017 0.94115
58 -.05416 -.01754 +.01031 0.93881
59 -.05624 -.01810 +.01083 0.93649

N 8

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Temp. Alteration of Volume from Heat. Alteration of Volume from Vapour. Increase of Density from Weight. Correct Spceific Gravity of satuated Air.
60 -.05833 -.01867 +.01114 0.93414
61 -.03041 -.01923 +.01146 0.93182
62 -.06249 -.01980 +.01178 0.92949
63 -.06458 -.02050 +.01217 0.92709
64 -.05666 -.02120 +.01256 0.92470
65 -.06874 -.02190 +.01295 0.92231
66 -.07083 -.02260 +.01334 0.91991
67 -.07291 -.02330 +.01372 0.91751
68 -.07499 -.02484 +.01457 0.91265
69 -.07708 -.02484 +.01457 0.91265
70 -.07916 -.02567 +.01503 0.91020
71 -.08124 -.02654 +.01551 0.90773
72 -.08333 -.02740 +.01598 0.90525
73 -.08541 -.02830 +.01648 0.90277
74 -.08749 -.02923 +.01699 0.90027
75 -.08938 -.03020 +.01752 0.89794
76 -.09166 -.03120 +.01810 0.89524
77 -.09374 -.03220 +.01861 0.89267
78 -.09583 -.03323 +.01916 0.89010
79 -.09791 -.03427 +.01973 0.88755
80 -.09999 -.03533 +.02030 0.88498
81 -.10208 -.03643 +.02090 0.88239
82 -.10416 -.03756 +.02150 0.87978
83 -.10624 -.03873 +.02213 0.87716
84 -.10833 -.03993 +.02277 0.87451
85 -.11041 -.04116 +.02343 0.87186
86 -.11249 -.04243 +.02411 0.86919
87 -.11458 -.04373 +.02486 0.86655
88 -.11666 -.04503 +.02549 0.86380
89 -.11874 -.04638 +.02618 0.86111
90 -.12083 -.04766 +.02688 0.85889

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To find the specific gravity of any mixture of air and aqueous vapour by means of this Table, we must proceed as follows:—Note the temperature and the point of condensation, by the hygrometer; if they coincide, that is to say, if the air be in a state of saturation, we shall find the specific gravity required in the fifth column, opposite to the proper degree of heat in the first column. If the point of condensation be below the temperature, we must look for the correction to be applied separately for the heat in the second column. The quantity to be subtracted, for the vapour of the given degree, must be sought for in the third column, and must be applied minus the quantity due to its weight, which stands beside it in the fourth.

For example:—If we wish to know the specific gravity of a mixture of air and vapour, of the temperature of 60°, and of which the dew-point is 40°, we find in the second column, opposite to 60°, the number .05833; which, deducted from 1.00000, leaves .94167. In the third column, opposite to 40°, we have .00934; and beside it in the fourth, .00580. Now .00934 - .00580 = .00354; which, subtracted from .94167, leaves .93813, as the number sought.

The application of this Table, to barometrical mensurations, is sufficiently simple. For this purpose, with the usual operations at the upper and lower stations, must be combined simultaneous observations of the dew-point, by means of the hygrometer; and the approximate height, deduced in the common way, may, at once, be corrected for temperature and moisture, by the specific gravity of the air so obtained. As the specific gravity of the air

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at the time of the experiment is to 1.00000, the standard, so will the approximate height be to the real height.

General Roy, from his own experiments, as well as from a careful review of those of Dr. Luc, made the medium expansion of air for 1° = .00245; and Sir George Shuekburgh assigned .00243 as the mean. These estimates, derived from barometrical experiments, made at different temperatures, and compared with known heights, must have included also the expansion due to the mean quantity of vapour; and upon reference to the last table, it will be found that the medium of the two combined effects is exactly .00244 for every degree of temperature; for .85839 deducted from 1.00000, leaves 0.14161, which, divided by 58, the total number or degrees from 32° to 90°, gives .00244.

General Roy, again, has fixed the point of temperature at which the specific gravity of mercury to the atmosphere is 10.435, at 33°; the average of his experiments, however, making it a little lower. Sir George Shuekburgh places it at 31¼. It is worthy of remark, that the point which approaches the nearest, by the table, to exact coincidence, is 31°; for at 32° the effects of temperature being null, a fall of one degree is necessary to neutralize the expansion of the vapour.

I shall now suppose a case, in which all the proper observations have been made, for the purpose of shewing more distinctly the manner in which I propose to apply the table of correction.

Barom. at lower station 20.528 Temp, of mercury 58°

upper ditto 88.161 51

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Deduct for temp. of 29.523
mercury .084
29.444 Logarithm 4689378
28.161
Ditto ditto .050
28.101 Logarithm .4437063
Approximate height in fathoms 202.315
× 6
Ditto ditto in feet 1213.890
Temp. of air at lower station 55 Dew-point 40°
Ditto ditto upper ditto 51½ Ditto 38
Mean 53¼

Expansion of air at 58¼ per Table .04374

1.00000
.04374
.95626 Sp. Grav. of air corrected for temperature.
Expansion of air for vapour at 40°, per table =.00934
Increase of Density for ditto 00580
.00354
Expansion of air for vapour at 38° 00880
Increase of density .00549
.00331
.00354
.00331
.00685
Mean .00842

And .95626 – .00342 = .95284 correct sp. gr. of atmosphere.

S.G. of air. Standard. Aprozt. height. Correct height.
Then .95284 : 1.00000 :: 1214 : 1274

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The hygrometer may be applied also to artificial atmospheres, and experiments upon confined air. Plate 1, Fig. 2, represents a receiver prepared for this purpose. A hole is drilled in its side, through which the tube proceeding from the ball within it, containing the thermometer, is passed, and welded with the tube proceeding from the other ball on its exterior, by means of a lamp; the stem is secured in the side of the glass with cement, the ether boiled, and the capillary opening closed, as before directed. The external ball is then to be covered with muslin; by this arrangement the evaporation from the latter produces a corresponding degree of cold upon the internal ball, which will measure the quantity of vapour included, by the precipitation, which may readily be marked. In delicate experiments a lighted taper, in a glass lantern, placed behind the bulb of the instrument, renders the deposition more easily visible, and ensures accuracy.

The hygrometric properties of any substance may thus be readily measured, by placing it under the receiver, and marking the absorption of the vapour.

Before I enter into the detail of experiments which I have made, during the last three years, with the hygrometer under consideration, the results of which I cannot help indulging a hope may be found to be interesting to science, I feel myself called upon, rather unwillingly, to say something of the merits of the instrument, in comparison with others which have been intended to answer the same purposes. I was induced, at first, to hope that the universally-acknowledged precision of the principle

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upon which its indications were founded, would have ensured for it a general adoption; and I hastened to communicate my observations in a more incomplete state than I should otherwise have done, from a conviction that the great value of the invention must be derived from the number, extent, and comparison of experiments performed with it by different observers, in different situations. I had, however, it seems, miscalculated the force of truth, and the vis inertiœ of prejudice and habit. Notwithstanding the motto of experimental philosophy, there are still those who are

"Addicti jurare in verba magistri."

and who wait for the ex cathedrâ fiat, to form their opinions*. Perhaps, however, I ought not to be surprised that so simple a contrivance should not

* Of the difficulty, to the uninitiated, of approaching the shrines whence the oracles of science are issued, some idea may be formed from the following circumstance.
Being actuated by the wish to obtain contemporaneous observations, and to do all in my power to facilitate so desirable an object, and my own opinion being confirmed by those whose judgment I could not doubt, I took an opportunity of sending, by a private hand, two of the hygrometers, in their most perfect state, to one of the Philosophers of the French Royal Academy of Sciences, the most distinguished for chemical knowledge and discoveries. I requested his opinion of the merits of the instrument, and authorized him to present one of them, in the most respectful way to the Academy. My presumption has, I suppose, been properly checked, by no notice whatever having been taken of what was certainly meant as a mark of humble respect, either by the individual, or the learned body: to the former of whom, having had the advantage of a personal introduction, I cannot feel that I have been to blame in addressing myself, however small may have been my pretensions for obtruding myself upon the latter.

[page] 186

have found its way across the channel to continental philosophers, when, as it appears, even the Tweed has been able to impede its progress in our own island. Professor Leslie, in an article upon meteorology, in the Supplement to the Encyclopœdia Britannica, published three years after my first paper oft the hygrometer, in one solitary remark upon the experiment of Le Roi, and the dew-ptfint, observes, "Could this method have been easily and nicely reduced to practice, it might certainly hate furnished an accurate estimate of the hygrometer, and state of the atmosphere."

I eagerly embrace this opportunity of recording this one more important testimony to the correctness of the principle, and have only to regret that the humble fame of the individual has been insufficient to attract the Professor's attention to means, which may surely be deemed easy and nice, of attaining an end, of the importance of which he is so thoroughly impressed. I am, doubtless, in courtesy, bound to suppose that Mr. Leslie would not be inclined to defend his encyclopædiacal knowledge at the expense of his candour.

It would never have occurred to me to enter into a comparison of my hygrometer with the hygroscopic contrivances which have been hitherto in use; for I conceived that the vagueness and fallacy of their indications, their gradual and necessary deterioration, their liability to derangement, and accidental injury, had been universally admitted; and I thought that an instrument to measure the portion of humidity which a given portion of air holds, or is capable of sustaining, had

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been an acknowledged desideratum in physics. But here again I was deceived. I have lately learnt, with no small degree of surprise, that there are some, even amongst those whose rank in science give weight and importance to their opinions, who prefer the observations of a hair, a gut, or the beard of an oat. The only reason assigned for this preference is the time which is occupied in taking an observation with my hygrometer, while mere inspection is sufficient to ascertain the indications of the others. To this I reply, that one accurate observation upon fixed and certain principles, is worth a thousand uncertain approximations; and that, therefore, if it were true that it requires much time for its management, the infallibility of the result should have ensured its adoption. But this waste of time must be a mere gratuitous assumption of those who never have fairly tried the experiment, for I speak from three years' experience, during which time I have used the instrument at least three times a day, when I assert that it requires less time to observe, in a proper manner, with the hygrometer, than with the barometer. To this we may also add, setting aside the uncertainty of the calculations, the time which is required to reduce the observations of the other instruments by formulaic processes to the dew-point, which must be performed before they can be applicable to any accurate purpose; a point at which we arrive at once with perfect precision by mere inspection.

The Editors of the Bibliothèque Universelle, of Geneva, in their Number for March, 1820, have given an account of my invention, and have detailed

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their reasons for preferring the hygrometer of their illustrious countryman De Saussure. In replying to their observations, I shall, it is to be presumed, answer the strongest statement that can be made in favour of hygroscopic substances.

In the first place, however, I am proud to record so strong a testimony to the accuracy of the combination as the following:—" On peut ne pas adopter toutes les théories de l'auteur ni partager sa prédilection pour l'appareil qui fait l'objet principal de son mémoire; mais on ne peut disconvenir que cet appareil, tel qu'il est construit, par M. Newman, fonctionne admirablement," The learned editors will pardon me, if I endeavour, by removing their objections, or rather their predelictions, to make them absolutely partake of my preference for the instrument.

"Il est à présumer," say they, "que l'auteur ne faisant mention nulle part dans son mémoire de l'hygromètre à cheveu, du feu De Saussure, n'en avait aucune connoissance; fait assez étrange vû la réputation qu'a acquise et que mérite à fort juste titre cet instrument pour toutes les recherches delicates. Il est pour le moins aussi sensible que celui de l'auteur; et pour la commodité du transport et de l'usage soit à l'air libre, soit en vases clos, l'hygromètre à cheveu l'emporte beaucoup. Il faut toujours faire une éxperience avec celui de l'auteur lorsqu'on veut connoitre l'état hygrométrique de l'air; il faut une provision d'éther, etc. Avec celui de De Saussure au contraire, il suffit de le regarder; en observant aussi le thermomètre dont les indications doivent toujours marcher pareillement à celles

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de l'hygromètre, ainsi que l'a prescrit soigneusement l'auteur dans son Essai sur l'Hygromètrie, l'un des fruits les plus remarquables de sa sagacité et de.son génie."

It would, indeed, have been strange, had the presumption been correct, that I was totally unacquainted with the instrument invented by that indefatigable philosopher. Long have I been an humble admirer of his sagacity and genius, and to no work have I been more indebted for useful instruction on the subject of which it treats, than to the Essay above referred to. My reason for not making mention of the hair-hygrometer of De Saussure was, as I have before stated, the conviction on my mind of the general admission of the inadequacy of any application of organic substances to the required accuracy of the purpose. I had selected one, as the best contrivance of this nature, to elucidate this point by contemporaneous observations with my own instrument; and the editors of the Bibliothèque Universelle, themselves, in recording my opinion " on verra combien ses indications sont vague et peu concluantes," add, " nous ne sommes pas très eloignés de cette opinion." Now, I must own; that I am quite at a loss to conceive any objection that can apply to the whale-bone, that does not equally affect the hair as an accurate measure of vapour. But I shall prefer supporting this conclusion by the authority of others, rather than by any arguments of my own; especially, as I think, that I can produce authority, which the candour of the editors themselves will allow to be conclusive.

And let us hear the Bibliothèque Universelle itself

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upon this very subject. First; as to the proper construction of the instruments.— "Il en est peu qui exigent autant qui l'hygromètre de l'adresse et des connoissances dans l'artiste qui l'entreprend; il devrait être physicien, mécanicien de main et même un peu chimiste. La facilité de se procurer la substance hygromètrique qui fait l'ame de l'instrument à tourné a piège: on a cru que partout où l'on pouvoit se procurer des cheveux on febriqueroit aisément les hygromètres; à la bonne heurs s'il s'agit d'hygromètres quelconque; mais on n'en obtient de réguliers, comparables, durables que de la main d'un artiste expérimenté." Next, as to its permanency and the consequent reliance that may be placed upon its indications.

"Dans ceux qui ont longtemps éprouvés les inclémences del'air, le cheveu acquiert un plus grande susceptibilité d'extension, et pour conservir à l'instrument une marche bien uniforme il seroit à propos de changer le cheveu tous les deux ans."— Bib Univ. Avril, 1819.

M. de Humboldt, the celebrated philosopher and traveller, who is equally distinguished by his aecuracy of observation, and by his philosophic generalizations, and who has had opportunities of making observations upon this subject which no other person ever yet enjoyed, and no other ever was more competent to appreciate, thus speaks of hygrometers in general, and of De Saussure's and De Luc's in particular*.

"We know, by very accurate experiments, the

* De Humboldt's Travels, translated by Helen Maria Williams. Vol. ii. p. 84, et seq.

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capacities of saturation of the air at different degrees of the thermometer; but the relations which exist between the progressive lengthening of a hygroscopical body, and the quantities of vapour contained in a given space, have not been appreciated with the same degree of certainty. These consideration have induced me to publish the indication of the hair qnd whale-bone hygrometers just as they were observed, marking the degree shown by the thermometers connected with these two instruments.

As the fiftieth degree of the whale-bone hygrometer corresponds to the eighty-sixth degree of the hair hygrometer, I made use of the first at sea and in the plains, while the second was generally reserved for the dry air of the Cordilleras. The hair, below the sixty-fifth degree of Saussure's instrument, indicates, by great variations, the smallest changes of dryness, and has, besides, the advantage of putting itself more rapidly into a state of equilibrium with the ambient air. De Luc's hygrometer acts on the contrery with extreme slowness; and on the summit of mountains, as I have often experienced to my greet regret, we are often uncertain whether we have not ceased our observations before the instrument has ceased its movement. On the other hand, this hygrometer, furnished with a spring, has the advantages of being strong, marking with great exactness, in very moist air, the least increment of the quantity of vapour in solution, and acting in all positions; while Saussure's hygrometer must be suspended, and is often deranged by the wind, which raises the counterpoise of the index, I have

[page] 192

thought that it might prove useful to travellers to mention in this place the results of an experience of several years." "Notwithstanding the doubts which have been raised in these latter times respecting the accuracy with which hair or whale-bone hygrometers indicate the quantity of vapours mingled in the atmotpheric air; it must be admitted, that even in the present state of our knowledge, these instruments are highly interesting to a naturalist, who can transport them from the temperate to the torrid zone, from the northern to the southern hemisphere, from the low regions of the air which rest on the sea, to the snowy tops of the Cordilleras."

"I have never been able to reduce the hair or whale-bone to the degree of extreme siccity for want of a portable apparatus, which I regret not having made before my departure. I advise travellers to provide themselves with a narrow jar containing caustic potash, quick-lime, or muriate of lime, and closed with a screw, by a plate on which the hygrometer may be fixed. This small apparatus would be of easy conveyance, if care were taken to keep it always in a perpendicular position. As under the tropics, Saussure's hygrometer generally keeps above 83°, a frequent verification of the single point of humidity is most commonly sufficient to give confidence to the observer. Besides, in order to know on which side the error lies, we should remember that old hygrometers, if not corrected, have a tendency to indicate too great dryness."

Mr. Leslie, in his Essay upon the relations of air to heat and moisture, makes the following remarks

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upon the same subject. "But these substances (viz., hygroscopic substances), especially the harder kinds of them, unless they be extremely thin, receive their impressions very slowly, and hence they cannot make with any precision the fleeting and momentary state of the ambient medium." "The expansion of the thin cross-sections of box, or other hard wood, the elongation of the human hair, or a slice of whale-bone, and the untwisting of the wild-oat, of cat-gut, of a cord or linen thread, and of a species of grass brought from India, have, at different times, being used with various success. But the instruments so formed are either extremely dull in their motions, or, if they acquire greater sensibility from the attenuation of their substance, they are likewise, rendered the more subject to accidental injury and derangement, and all of them appear to lose, in time, insensibly, their tone and proper action."

But it is to the Essay of M. de Saussure himself, that I appeal with the most confidence, for the confirmation of this opinion. It is replete with acknowledgments of the obvious defects of instruments constructed upon this principle; defects which it was impossible that a mind like his could overlook or attempt to conceal: and it proves fully that his sagacity and genius were tasked to the utmost, to diminish the sources of uncertainty which it was out of his power wholly to remove. Any person, who had not seen the minute instructions given by this able philosopher for the construction of his hygrometer, would be surprised at the nicety required in its adjustment. The mere preparation of the hair is a process of great delicacy and uncer-

O

[page] 194

tainty. It is previously exposed to an alkaline lixivium, upon the due strength and regulated application of which depends its most valuable properties; and hairs which have been unequally exposed to this action are no longer fit for comparison with one another. "Les cheveux n'ont une marche parallèle que quand ils sont également lessivés." So that it would be impossible for an artist in London, although he were " physicien, mécanicien de tête et de main, et même un peu chimiste" with the most scrupulous attention to the directions contained in this Essay, to construct an instrument which should range with one made in Geneva, unless he had the means of actual comparison.

But after all the care which the ingenuity of such a philosopher could devise (and none but such a philosopher could be competent to take such precautions), thus guardedly and candidly does the inventor speak of the best instruments, " Quant à la comparabilité des hygromètres construits avec cette substance je puis dire que deux ou plusieurs de ces instruments, faits avec des cheveux sem-blablement préparés, gradués sur les mêmes principes, et exposés ensuite aux mêmes variations d'humidité et du sécheresse, ont des marches que l'on peut nommer parallèles. Je ne dirai cependant pas qu'ils indiquent toujours tous le même degré, mais que leurs écarts vont rarement audelà de deux degrés. Si après que deux hygromètres auront séjournes, pendant long-temps dans un air très sec, par exemple, au quarantième degré de ma division, on en porte un dans un air encore plus sec, qui le fasse venir, je suppose, à trente, et que pendant ce

[page] 195

temps-là, l'autre ait été porté dans un air un peu moins sec, par exemple à cinquante degrés; qu'ensuite on les replace tous les deux dans l'air où ils ètoient d'abord, ils ne reviendront ni l'un ni l'autre à quarante; celui qui vient de l'air le moins sec restera à quarante-deux ou quarante-trois; et celui qui vient de l'air le plus sec ne montera qu'à trente-sept ou trente-huit." "Cet Hygromètre a l'inconvénient de ne pas revenir bien exactement au même point lorsqu'on l'agite un peu fortement, ou qu'on le transporte d'un lieu dans un autre, parceque le poids de trois grains qui tient la lame d'argent tendue, ne peut pas la ployer assez exactement pour la forcer à se coller toujours avec la même précision contre l'arbre autour du quel elle se roule: or on ne peut pas augmenter sensiblement te poids sans des inconvéniens plus grands encore. D'ailleurs si le cheveu est trop long, le vent, lorsqu'on observe en plein air, a trop de prise sur lui, et communique ainsi à l'aiguille des oscillations incommodes."

The relation of the degrees of this hygrometer, to the actual quantity of vapour in the air, is moreover very fer from having been determined; "C'est ce que j'ai tenté de faire," says the inventor, "pour mon hygromètre; mais on verra que ce travail difficile est encore bien loin de sa perfection."

When we add to these admissions the disturbing influence of heat, which is so great, that the mere approach of the hand causes a sensible movement towards dryness; the adhesion of dust and spiders webs; the choaking of the pivot of the wheel; and

O 2

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the possibility of friction from the index; we shall have some notion of the sources of error in this instrument, which the great philosopher, its inventor, himself, has pointed out and laboured to modify.

It is thus that I reply, or rather it is thus that universal experience replies, to the "pour le moins aussi sensible," of the editors of the Bibliothèque Universelle. As to the "Commodité du transport et de l'usage," I must remark, that the whole of the new apparatus packs in a box, which may very conveniently be carried in the pocket; and although each observation with it may, in strictness, be called an experiment, yet that infinitely less time is required to make this experiment, than would be necessary to assure an observer, with either the hair or whale-bone hygrometer, that "the instrument had ceased its movement." The inconvenience of carrying a supply of ether, may, I think, fairly be set against that of an apparatus for rectifying the instruments described by De Humboldt, and which he considers necessary to give confidence in their indications.

But upon this point I shall avail myself of direct evidence of the most unexceptionable nature.

Mr. Caldcleugh, in his "Observations in Brazil, and on the Equator," (Jour. Royal Inst. vol. xiv. p. 46,) remarks, "When I commenced using the instrument, I was almost afraid to touch it, from its apparent delicacy, but was soon convinced, from the many rude shocks it underwent, that it was stronger than I had imagined; more than common careless-

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ness, indeed, is required to break it. I may be permitted to add, that I think no traveller will find any inconvenience from carrying this hygrometer or its accompaniment, a small stock of ether; the latter I usually placed among my linen."

Captain Sabine, also, after twelve months' experience between the tropics, during which time he daily made numerous observations, thus bears testimony to the same fact:

"I have great pleasure in remarking, that I found much less difficulty than I had anticipated in getting corresponding observations made with the hygrometer, on the correctness of which I could depend; the ingenuity in the principle of this instrument, and the simplicity of its application, together with the decisive nature of the results which it gives, independent of the labour, and, at best, the uncertainty of formulaic deduction, form its great advantage over the methods by evaporation, or the indications of hygroscopic substances: these particulars excite an interest in its trial, in persons to whom it was previously unknown, which is probably the reason that the distrust, which is almost always in the first instance expressed, of precision in the observation itself, is found to give way in practice so much sooner than might be supposed. It may be useful, also, to travellers in warm climates, to add a remark from my own experience, that in ascending elevations, or in journeying inland over rough roads, the ether carries perfectly well in a bottle in the waistcoat-pocket with a common cork capped with leather; and that the expenditure of ether altogether will probably fall much short of

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the estimate, as, with ordinary care, very little will be wasted." (Journal Royal Inst. vol. 15, p. 71.)

The quantity of ether expended by Captain Sabine during the year, fell something short of a pint.

The instrument, which I have presumed so strongly to recommend, shall be still further judged by the very competent authority to which I have been referred. M. de Saussure sums up, in his Essay, the qualities which a perfect hygrometer ought to possess; allowing, candidly, that his own falls very short of the perfection which he proposes. All I would ask is, if the one which I have invented fulfil all the conditions laid down as follows, that for the good of science it may be adopted as a standard by experimental philosophers.

"Un hygromètre seroit parfait: premièrement si les variations étoient assez étendues pour rendre sensibles les plus petites différences d'humidité, et de sécheresse.

2. Si elles étoient assez promptes pour suivre pas-à-pas toutes celles de l'air, et pour indiquer toujours exactement son état actuel.

3. Si l'instrument étoit toujours d'accord avec lui-même, c'est-à-dire, qu'au retour du même état de l'air il se retrouvât toujours au même degré.

4. S'il étoit comparable, c'est-à-dire, si plusieurs hygromètres construits séparément sur les mêmes principes indiquoient toujours le même degré dans les mêmes circonstances.

5. S'il n'étoit affecté que par l'humidité ou la sécheresse proprement dites.

6. En-fin si ces mêmes variations étoient pro-

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portionnelles à celles de l'air, en-sorte que dans des circonstances pareilles, un nombre double ou triple de degrés indiquât constamment un quantité double ou triple de vapeurs."

But there is yet another method of estimating the quantity of moisture at any time existing in the air, with which my own has been brought into comparison, and which is not liable to the same class of objections as those which we have been just considering; I mean that of a comparison between the temperatures of a moist and a dry thermometer. Dr. Hutton was the first who conceived the idea of applying such an observation to the purposes of hygrometry. " I used to amuse myself," says he, " in walking in the fields, by observing the temperature of the air with the thermometer, and trying its dryness by the evaporation of water. The method I pursued was this: I had a thermometer included within a glass tube, hermetically sealed: this I held in a proper situation until it acquired the temperature of the atmosphere, and then I dipped it into a little water, also cooled to the same temperature. I then exposed my thermometer, with its glass-case thus wetted, to the evaporation of the atmosphere, by holding the ball of the thermometer, or end of the tube in which the ball was included, towards the current of the air; I examined how much the evaporation from that glass tube cooled the ball of the thermometer which was included."

Now this simple observation, it is probable, may furnish the necessary data for solving the problem in all its particulars: but if it do, it is by means of abstruse calculations, and many delicate corrections,

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upon the nature of which philosophers are toy no means agreed.

Mr. Leslie has founded upon it an instrument, upon which he has expended much ingenuity and research: it consists of an air-thermometer, one of the balls of which is covered with muslin, and kept moist; but, in departing from the simplicity of the original expepment, he has multiplied the sources of error, both in construction and observation. He has, moreover, substituted an arbitrary scale for that of the common thermometer; and his hygrometer possesses a deceptive sensibility, which is liable to be affected by more causes than those which can be taken into account.

The observation, however, in its most simple and unexceptionable form, is by no means so easy to make with accuracy, as might, at first sight, appear. It is almost impossible to take the heat of the air to any degree of nicety, without the observation being affected by the power of radiation; and, if radiant caloric be allowed to interfere, the conditions of the calculation fail. The temperature of evaporation is no longer that constant quantity which it is supposed to be, if dependant only upon the temperature of air, and is liable to fluctuations with every change of place, and every breath of wind. The density of air must be also taken into the account, and it is allowed that the cold produced by evaporation from the moistened bulb of a thermometer, must depend, in some measure, upon the height of the barometer.

It would be foreign to my purpose here to enter into the theory of the experiment, which would itself be a work of much length: it is sufficient to observe

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that such men as Mr. Leslie*, Mr. Anderson †, Mr. Ivory ‡, and M. Gay-Lussac §, after having bestowed immense labour upon the subject, all differ from one another most essentially in their formulae, corrections, and final results; and the hygrometer by evaporation, has not been able to save Mr. Leslie from the conclusion that hydrogen gas " must, in similar circumstances, hold in solution seven times as much moisture as the atmospheric medium; " or Mr. Colebrooke||, from the probable error of placing the dew-point on the coast of Africa, below 0° of Fahrenheit's scale. I cannot better dismiss the subject than in the words of M. Gay-Lussac,—

"En général on peut parvenir à connaître l'état hygrométrique de l'air, d'après le froid produit par l'évaporation; mais comme ce froid est variable avec la pression de l'air, sa température, son degré d'humidité, il faudrait des tables très-étendues pour le déternjiper avec exactitude. J'avais voulu entreprendre ce travail, en répétant mes expériences sur le froid produit par l'évaporation, et en faisant de nouvelles; mais j'ai été rebuté par sa longueur, et le défaut de données suffisamment exactes, et surtout par la considération que l'ingénieuse procédé de Le Roy était susceptible d'une application plus facile et que, dans l'état actuel de la Physique, il étoit de beaucoup préférable. "—Ann. de Chim. tom. 21, p. 91.

* Supp. Ency. Brit,, Art. Meteorology.

† Edin. Ency., Art. Hygrometry.

‡ Phil. Mag., vol. lx., p. 81.,

§ Ann. de Chimie, tom. xxi.; p. 82.

|| Journal of the Royal Institution, vol. xxvii, p. 115 et seq.

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The observations of the hygrometer with which I have been chiefly engaged, relate principally to meteorology; and I conceive that it will be better to reserve such remarks as I have to make upon them for another Essay, than to enter upon their discussion in this place. I shall, therefore, conclude this paper by detailing a few experiments only, which may either throw some light upon the action of the instrument, or may tend to facilitate its use.

Exp. 1.—With the thermometer at 60°, I found the point of condensation to be 50°: I then took a receiver, fitted with a hygrometer, and ground to the plate of an air-pump, whose capacity was fifty-six cubic inches. The condensation was produced very visibly under the glass at the same temperature. Now the quantity of vapour in a cubic foot of air, under the above conditions, was only 4.445 grains; therefore the quantity actually included in the receiver, could only be 0.144 grains; which will serve to prove the extreme delicacy of the instrument, as it distinctly indicated so small a quantity. The receiver was then, without changing its contents, slid over a vessel containing water. In an hour and a half, the external temperature remaining the same, the precipitation took place at 57°. At the expiration of another hour and a half, the affiision of ether upon the exterior ball, caused instantaneous condensation upon the interior one, shewing that saturation, at the existing temperature, had taken place. The bell-glass was now slid from the water, and placed over a glass containing a few drops of sulphuric acid. After re-

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maining a quarter of an hour in this situation, a depression of temperature of 30° produced no mist upon the instrument.

Exp. 2.—The receiver was placed upon the plate of the air-pump with some water under it; the air was then exhausted as perfectly as possible. The barometer stood at 29.79 inches, the thermometer at 62°; the gauge of the pump at 29.20; to the latter should be added the pressure of the included vapour at 62°=.59 inch, which would make the gauge and the barometer exactly correspond. When ether was dropped upon the exterior ball, precipitation was instantaneous. Air was now admitted gradually, till the gauge fell to 14 inches: the point of condensation was not altered, neither was it affected by restoring the equilibrium completely.

Exp. 3.—Temperature 64°; point of condensation 61°. The air in the receiver was rarefied till a copious cloud was formed; the gauge then stood at 8.1 inches, and the point of condensation had fallen to 54°. When the glass had risen to 60°, the air was suddenly restored, and a copious dew was formed upon it; the exhaustion was next carried on, till the cloud which was formed had totally disappeared, and the gauge stood at 24.2 inches. No precipitation took place at a temperature of 34°; the air was gradually re-admitted, and the deposition took place with the hygrometer at 36°, and the gauge at 15 inches.

Exp. 4.—The receiver was filled with oxygen in contact with water, and afterwards with hydrogen; but the point of condensation was the same as

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when filled with common air, under the same circumstances. This, with Experiment 2, fully coincide with Mr. Dalton's view of the theory of mixed elastic fluids, and prove, indeed, that the gases are as vacua with regard to vapour; and that, where they happen to be mixed together, they exist as independent atmospheres.

Exp. 5.—Having absorbed all the vapour contained in the receiver by means of sulphuric acid, I placed it over some spirits of wine; after remaining some time in this situation, a few drops of ether upon the hygrometer produced an instant precipitation. The experiment was also made with ether, in the place of the spirits of wine, with the same results.

Exp. 6.—The temperature of a room being 45°, I found the point of condensation in it to be 39°. A fire was lighted in it, the door and windows carefully shut, and no one was allowed to enter: the thermometer rose to 55°, but the point of condensation remained the same. A party of eight persons afterwards occupied the room for several hours, and the fire was kept up: the temperature increased to 58°, and the point of condensation rose to 52°.

I must now refer to the subjoined journals for further exemplifications of the application of the hygrometer, and to the Essays upon the Constitution of the Atmosphere, and upon the Climate of London, for such general theoretical conclusions as I have thought myself entitled to draw from my own extended observations, and the kindly-communicated observations of my friends.

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Whatever opinion may be formed of my reasoning upon these subjects, let the hygrometer be judged upon its own merits alone; and if it shall be found to be liable to no errors of construction, and no deterioration, from use or age; if its indications shall prove to be infallible, and strictly comparable, under all circumstances; and if, moreover, it be easy to observe, and its observations applicable without the trouble and uncertainty of formulaic calculations, I shall still hope, that, for the good of science, it may be generally adopted.

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AN ESSAY

UPON THE

RADIATION OF HEAT IN THE ATMOSPHERE.

THE radiation of heat is a subject of the utmost interest and importance, and the difficulties which surround it have exercised the ingenuity and industry of some of the greatest philosophers of modern times. Count Rumford, Professor Leslie, Sir Humphry Davy, Professor Prevost, Dr. Delaroche, and M.M. Du Long and Petit, have particularly distinguished themselves by their experiments and reasonings upon it; and the latter gentlemen, more especially, have demonstrated some of the laws of the distribution of heat, with mathematical precision.

With regard to the influence of this power upon the œconomy of nature, however, but little is at present known; and the elegant and demonstrative Essay of the late Dr. Wells upon the formation of dew, stands almost alone in exhibiting its important connexion with the welfare of the vegetable kingdom. His successful labours were directed to one particular branch of, what would appear to be, a very extended inquiry; for there can be little doubt that radiant caloric must have a direct and very important influence upon many of the processes of vegetation.

It is with a view of exciting some attention to a

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subject which appears to me to be so well worthy of elucidation, and to suggest some experiments, which, to render them beneficial, require much perseverance and extensive co-operation, that I venture to bring forward some observations of my own, which, I am sensible, are in a very imperfect state; but to which I have devoted much attention during the last three years. I hesitate the less to do so, as I am enabled, by the kindness of my friend Captain Sabine, (whose zeal for science prompted him, amidst the laborious operations connected with more important objects, to devote much of his leisure time to the study of atmospheric phenomena,) to give them additional interest, by combining with them his experiments in tropical climates.

It has often struck me with surprise, that, in the numerous meteorological registers which are published in different parts of the world, no one has ever thought of including observations upon the intensity of the solar rays at different seasons of the year, and in different situations. It is well known to the agriculturist and the gardener, that without the direct influence of the sun, whatever may be the temperature of the air, the fruits of the earth seldom come to perfection. What, then, is the force of this important agent? what the modifications to which it is subject? and how is its energy spent, when screened by concrete vapours from the surface of the earth? Does its influence increase with the temperature of the air from the pole to the equator? or is the rapid vegetation of the arctic regions, during the short summer of those climates, dependant upon any compensating energy of its operation?

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Before I attempt to answer these questions,. I will propose another, which, many will be surprised to find, cannot be met with an immediate solution; which is, the maximum degree of heat to which a plant, or the parts of a plant, are subjected by exposure to a mid day sun at midsummer in this climate?

Many persons have, at different times, exposed naked thermometers to the direct light of the sun, and marked their rise; but such trials have never been persevered in, or registered with any exactness. Nor were the means employed calculated to resolve the problem with any precision. Few of the rays would impinge directly upon the bulb of the instrument so placed, and all, but the direct rays, would be reflected from it. The results would necessarily vary with size and shape, and no two thermometers would, probably, agree in their indications.

There are, no doubt, in all the plants of the vegetable kingdom parts which are calculated to absorb all the radiant heat which strikes upon them; and therefore it is desirable to know, with a reference to this subject alone, the utmost amount of temperature which radiant matter is capable of producing.

My meteorological register includes a column for observations upon this point. They are complete from November 1820, to the end of December 1821, and from the beginning of May 1822, to the end of August of the same year. They were made by means of a register thermometer of large range, having its bulb covered with black wool, and placed upon a south border of garden-mould, with a full exposure to the sun. The thermometer did not rest

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upon the earth, but was supported about an inch above it. The arrangement was by no means unobjectionable, but the irregularities, to which it was liable, would, it was hoped, be, in a great measure, balanced by the multitude of the observations. The maximum heat of the sun's rays during the day was thus measured and entered in the journal. The following Table presents us with the average intensity of the solar radiation for every month in the year, orthe mean greatest height of the black thermometer above the surrounding medium, together with the utmost intensity observed in the same periods. The first column exhibits the month, the second the mean maximum temperature of the air, the third the average effect, and the fourth the maximum energy of the sun's light.

TABLE I. Shewing the mean maximum Temperature of the Air, with the mean and maximum Power of the Sun, for every Month of the Year.

Mean Maxim. of the Air. Mean Maxim. Force of Solar Radiation Maximam Force of Solar Radiation
January 39.6 4.4 12
Feburary 42.4 10.1 36.
March 50.1 16. 49.
April 57.7 28.1 47.
May 62.9 30.5 57.
June 69.4 39.9 65.
July 69.2 25.8 55.
August 70.1 33.1 59.
September 65.6 32.7 54.
October 55.7 27.5 43.
November 47.5 6.7 24.
December 43.2 5.4 12.

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Hence it appears that, as it might have been predicted, the power of solar radiation follows the course of the sun's declination. The maximum intensity and-effect oecurin June, while the greatest mean temperature of the atmosphere does not take place till July. This arrangement, no doubt, has an important influence upon the processes of fructification in the vegetable kingdom. Agriculturists are well aware of the advantage of direct solar heat in the flowering of wheat, and other corn-crops; an advantage which is never compensated by any elevation of temperature under a clouded sky. A table, similar to that above given, founded upon the experience of several years, would furnish a very valuable standard of comparison, and the causes of fruitful and unfruitful seasons would, no doubt, be found to be intimately connected with the particulars of which it would be composed. For example, it will be seen in the register, that in the very fruitfid year of 1822, the force of the sun's radiation in May was 7°, and in June 5°, above the corresponding months of the year 1821, in which the crops of corn were universally blighted and mildewed. The discordances above exhibited would also be found to vanish in a more extended average, and a more regular progression would be elicited from the balance of disturbing causes.

I have also been at some pains to ascertain the progression of radiation from the sun from its rising to the meridian, and from the meridian to its setting. The following are the details of a series of observations, made for this purpose in the month of June, 1822. The day was perfectly calm and cloudless,

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and the atmosphere so clear that the disc of the moon was visible throughout the day. The dew-point, by the hygrometer, was stationary at 57°, and only a few light cirri were discernible in the south-east quarter of the heavens.

TABLE II. Shewing the Progress of Solar Radiation from Morning to Evening.

Thermometer.
Time. In Sun. In Shade. Difference.
A. M. 9 93 68 25
103 69 34
10 111 70½ 40½
10½ 119 71 48
11 124 71½ 52½
11½ 125 72½ 52½
12 129 73 56
P.M.0½ 132 74 58
1 141 74½ 66½
140 75 65
2 143 75½ 67½
138 76 62
3 138 76½ 61½
132 77 55
4 124 76 48
123 77 46
5 112 76 36
106 75 31
6 100 73 27
Means 124¾ 73½ 51¼

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The mean results of five series of experiments, conducted with every possible precaution, are contained in the following table, shewing the power of the sun's radiation from 9½ A. M. to 6½ P. M., in the month of June.

TABLE III. Shewing the Progress of the Solar Radiation from Morning to Evening in June, upon an average of five Experiments.

Time. Force of Sun's Rays.
9½ A.M. 32
10½ 46
11½ 55
12½ 63
1½ P.M 65
63
58
49
35
29

Some important questions now present themselves, which the present state of our knowledge will not, I fear, enable us to answer satisfactorily. It may, however, be of some use to point them out as objects of future investigation. As the mean effect of the sun's radiation upon the surface of the earth falls so much short of the impression which it is capable of producing, in what way is its energy spent? Is it absorbed or dissipated in mid air? How is the mean temperature affected by it? How does it

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modify the ascending gradation of temperature in the atmosphere? What is its influence upon the concrete vapour of the clouds? Is it not the source of partial and very unequal expansions and cantractions in the aërial medium?

Before I proceed to detail some experiments which are connected (but sligtly, I fear,) with these interesting considerations, I shall avail myself of Captain Sabine's kindness, to institute a comparison of the radiant power of the sun in different latitudes.

The first series of experiments to which I shall refer, were made in March, 1822, at Sierra Leone. The general state of the atmosphere was as follows:—

"The day commenced as usual, calm and clear; between a quarter and half-past ten the sea-breeze sprung up from the N.W., freshened at noon from the W.N.W., accompanied by a diffused haze. At one, P.M., cleared, the wind still freshening. At two, some very light clouds in the zenith, which clearing away before three, it became hot and oppressive in the sun, the sea-breeze gradually declining towards evening, and the land-wind setting in at half-past nine."

Observations were made with the thermometers now described.

No. I. The mean of two thermometers, one with a silvered, the other with a blackened bulb, (differing never more than two or three tenths of a degree), freely suspended in the thorough draft in a store-house with open doors and windows every way, and with a veranda around open at the sides.

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No. II. A thermometer with a blackened bulb, suspended freely between the pillars of a transit instrument (the instrument itself being removed), about one and a half feet above the earthen parapet of the fort, and several feet above the general level of the ground, in a fair exposure to the sun and wind.

No. III. A similar thermometer to the preceding in the same exposure, but in vacuo in a glass case.

No. IV. A thermometer in the same glass case with the preceding, but having its bulb enclosed in a double case of polished silver, not in contact with the glass or bulb.

No. V. A differential thermometer in vacuo; the sentient ball coloured dark, and the other enclosed in a double case of polished silver. The graduation of this instrument was on the millesimal scale; i. e., the interval between the boiling and freezing of water, divided into 1000°, The three last were only exposed at the time of observation, being removed intermediately into the house, when the equilibrium in the bulbs of No. V. was gradually restored at the temperature indicated by No. I. These thermometers were registered, when the effect of the exposure had reached the maximum.

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TABLE IV. Experiments upon Solar Radiation at Sierra Leone.

March the 2d.
Time. No.1. No.2. Difference. No.3. No.4. Difference. No.5. OBSERVATIONS.
A.M.10 79.3 95. 15.7 110 70
11 80. 93. 13. 109 78
12 80.2 91.5 11.3 105 82 Haze.
P.M. 1 81.1 88. 6.9 109 102 7. 81 More clear, and wind freshening.
2 80.9 85. 4.1 109.5 102 7.5 64 Light clouds.
3 83.4 91. 7.6 118 107 11. 70 Clear.
4 82.9 90. 7.1 116 108 8. 53 Wind dying away.
5 81.4 83.5 2.1 100 98 2.
80. 81.5 1.5 86 86 0.

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The same thermometers were employed on the 4th of March, the variations of the weather being as nearly similar as can be described—the usual weather, in short, of the season.

TABLE V.—Experiments upon Solar Radiation at Sierra Leone.

March 4.
Time. No.1 . No. 2. Differ. No.3. No.4. Diffe. No. 5
A.M.9 80 95 15 110 102 8 69
10 80.5 93. 12.5 110 105 5 79
11 80. 94 14. 110 105 5 86
12 80.2 98.5 18.3 115 108 7 93
P.M.1 80.8 96. 15.2 118 108 10 88
2 81. 97. 16. 118 110 8 77
3 83 90.5 7. 5 113 105 8 69
4 82.5 89.5 7. - 109.5 102 7.5 59

The first, and most striking, result of these observations, is the very small comparative energy of the solar rays. All the means adopted to measure their effect concur in this conclusion. The utmost difference between a blackened thermometer in the sun and another in the shade was only 18.3°; and in vacuo y of one prepared to repel the radiant heat, and of another to absorb it, 11°. It is obvious that the whole difference between the first and third thermometer cannot be ascribed to radiation, for the latter, although placed in a rarefied medium, was still surrounded by attenuated air and aqueous vapour, the latter of which appeared in a pretty

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copious precipitation of moisture upon the sides of the glass. This atmosphere, however rare, was liable to become considerably heated under confinement. But even the maximum of this difference is only 38°.

The next series of experiments, which were made at Bahia, on the coast of Brazil, come into more immediate comparison with my own, and agree in the conclusion of the diminution of the force of radiation from a tropical sun. A mercurial register thermometer, having its bulb blackened and covered with black wool, was fully exposed to the sun on grass, and compared with a thermometer in the shade: the following Table exhibits the results.

TABLE VI.—Experiments upon Solar Radiation at Bahia.

Sun. Shade. Difference.
July 24 114 82 32
25 123 82 41
26 124 83 41
27 123 83 40
28 95 78 17
29 115 78 37
30 127 80 47

Here the maximum effect was only 47°, with a nearly vertical sun; while the same influence, in our temperate climate in June, in a medium not much, less heated, was 65°.

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Captain Sabine instituted a third set of experiments, upon the same point, during his stay in the Island of Jamaica.

The thermometer, with black wool, was exposed to the sun on the vegetation by which Port Royal is surrounded. It is a tongue of sand, projecting a considerable distance into the sea, and overrun by the Tibullus Maximus, which was at the time in flower. The ball of the thermometer was in contact with the vegetation, and supported by it about ten indies off the ground.

TABLE VII.—Experiments upon Solar Radiation at Jamaica.

Sun. Shade. Difference.
Aug. 25 122 86 36
26 123 87 36
27 122 86 36
28 122 86 36
29 123 86.5 36.5
30 123 86.5 36.5

A naked mercurial thermometer, suspended freely across between the upper branches of a stunted dead acacia, and exposed to the sun, near the other thermometer, about four and a half feet above the ground, and not in contact with the tree, carefully observed at intervals of the fore and after-noon, from the 25th to the 30th of October, was never seen to rise higher than 92°. This point it usually attained at ten A.M., before the sea-breeze set in,

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fell as the breeze commenced, but attained about the same height in the afternoon, although the breeze had freshened intermediately.

The indications of the differential thermometer, before described in vacuo, were as follow:

TABLE VIII. Experiments upon Solar Radiation, at Jamaica.

Date. Hour. Differential Thermometer. OBSERVATIONS.
Oct. 24 12 88 Strong breeze—perfectly clear.
P.M. 1½ 88 Ditto ditto
26 A. M. 9½ 82 No breeze—very clear.
12 88 Fresh sea breeze—in 2 minutes.
29 12 90 Clear—little breeze—in 2 minutes.
P. M. 2 86 Ditto—more breeze—in 2 minutes.
30 A.M. 10 88 Very clear—no breeze—in 2 min.
P.M. ½ 91 Very strong sea breeze—in 2 min.
74 Very strong breeze—in 2 minutes.
Nov. 3 A. M. 8 68 Calm and clear.
9 82 Ditto ditto
7 A. M. 7 48 Ditto ditto

The smallness of the effect is no less striking from these results than from the last.

In looking over the interesting personal narrative of M. de Humboldt, (in which the inquirer, upon almost any subject, is sure to meet with valuable information) I find ample confirmation of these observations. At Cumana he remarks, " I have often endeavoured to measure the power of the sun, by two thermometers of mercury perfectly equal, one

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of which remained exposed to the sun, while the other was placed in the shade. The difference resulting from the absorption of the rays in the ball of the instrument never exceeded 3°. 7 (6°. 6 Fahr.) Sometimes it did not even rise higher than one or two degrees.—Humboldt's Travels, by H. M. Williams. Vol. ii. p. 58.

A fourth set of Captain Sabine's experiments, in the mountains of Jamaica, present a comparison of the greatest interest. The observations were made on the 31 st of October, at Mr. Chisholm's house, situated on the summit of the Port Royal ridge, 4,000 feet above the sea. The woolled thermometer was laid upon the grass-plat about 100 yards from the house, and fairly exposed to the sun. In the forenoon, in intervals of the breeze, when the sky was clear, it rose above 130°; the thermometer in the shade, at the time being, 73°. The difference of 57° exhibits a much greater intensity of action than any that had been obtained at the level of the sea. The following observations of the differential thermometer in vacuo, in the same situation, confirm the conclusion.

TABLE IX. Experiments upon Solar Radiation, upon the Mountains of Jamaica.

Date. Time. Differential Thermometer. OBSERVATIONS.
Octr. 31. A. M. 9 74 1½ Minute.
10 100 1 ½ Minute.
11 84 Maximam produced in 1 ½ Minute, light clouds.
12 100 1 ½ Minute, very clear.

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As it appears, I think, incontrovertible, from a comparison of Captain Sabine's experiments with my own, that the force of the sun's direct radiation decreases in approaching the equator, I became anxious to ascertain, if possible, whether an analogous contrary effect were observable in advancing towards the pole. In looking over the journal of the late Expedition for the discovery of a north-west passage, I found some observations, which tended much to establish this curious fact. At: page 157 of that interesting narrative, Captain Parry observes—" On the 16th, (March), there being little wind, the weather was again pleasant and comfortable, though the thermometer remained very low. While it continued, nearly calm, we observed the following differences in the temperature of the air in the shade, and in the sun; the latter were, however, noted by a thermometer placed under the ship's stern, which situation was a warm one, for the reasons before assigned." The difference of warmth in this situation had been before ascertained not to exceed 2° to 5°.

TABLE X. Experiments upon Solar Radiation at Melville Island.

Date. Time. Sun. Shade. Difference.
March 16 A.M. 9 + 24 -24 48
10 + 27 -23 50
11 +28½ -22 50½
12 +29 -21 50
P.M.3 + 19 -13 32

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Again, on the 25th of March, the thermometer in the son being placed at a distance from the ship, and the weather very fine and calm.

TABLE XI. Experiments upon Solar Radiation, at Melville Island.

Date. Time. San. Shade. Diffcrence.
March 25 12 + 30 – 25 55
P.M. l +17 – 22 39
2 + 25 – 22 47
3 + 21 – 22 43

Here it is seen that the sun had power to raise a thermometer, which had not been prepared to receive its greatest impression, 55° in the month of Mardi, at Melville Island; the maximum effect in the vicinity of London, in the same month, upon a thermometer covered with black wool, being only 49°.

In Captain Scoresby's " Account of the Arctic Regions," there are also many remarks which powerfully confirm the same opinion. " The force of the sun's rays," he observes, " is sometimes remarkable. Where they fall upon the snow-clad surface of the ice or land, they are, in a great measure, reflected, without producing any material elevation of temperature; but when they impinge on the black exterior of a ship, the pitch on one side occasionally becomes fluid, while ice is rapidly generated at the other; or, while a thermometer placed against the black paint work, on which the sun shines, indicates a temperature of 80° or 90°, or, even more, on the opposite side of the ship, a cold of 20° is some-

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times found to prevail. This remarkable force of the sun's rays is accompanied with a corresponding intensity of light."—Vol. I. p. 378.

To ascertain with precision the temperature denoted in the above extract, by the pitch becoming fluid, which appears to me to furnish the best measure of the force of the sun's rays, I tried the following experiment. I covered the bulb of a thermometer with pitch to the thickness of about 1/10 of an inch, and suffered it to remain till it had become quite hard. I then held it at some distance from a fire, and noted the following points. At 100° Fahr., it began to soften. At 110°, it might be moulded into any form—and, from 120° to 136°, it rapidly approached fluidity, and, at the latter temperature, dropped off the ball. The degree denoted cannot, therefore, be placed lower than 120°; and if ice were forming at the same time in the shade, the force of the sun's radiation could not be less than 90°.

In the account of Captain Scoresby's last voyage to Greenland, a direct experiment in latitude 80° 19', confirms the same conclusion.

"The sun broke through the clouds, and produced a powerful effect upon the temperature. At two, A. M., the thermometer was 3° or 4° below zero. At eight, A. M., it was + 6°, and at ten, A. M., about + 14° in the shade. But the genial influence of the sun was still more striking. In a sheltered air, it produced the feeling of warmth; the black paint work of the side of the ship, on which the sun shone, was heated to a temperature of 90° or 100°, and the pitch about the bends became fluid. Thus, while on one side, was uncommon

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warmth, on the opposite side, was intense freezing." Journal of a Voyage to the Northern Whale Fishery, p. 34. The radiating force of the sun must, therefore, have been 80° in the month of April.

With respect to the greater energy of the solar rays, upon the summit of a mountain, than upon a plain, I find that Monsieur de Saussure made some decisive experiments, which establish the same feet. In his "Voyages dans les Alpes," the following observations occur—" Je cherchais à Génève un verre ardent assez petit pour qu'il n'eut précisément que la force nécessaire pour allumer de l'amadou. Je portai en suite le même verre et le même amadou sur le haut de Salève et je le vis là produire le même effet que dans la plaine et même avec plus de promptitude." Tome II. p. 363.

"Sur la cime du Cramont (777 French toises = 4967 English feet above the plain) un thermomètre appliqué sur le liège noirci, exposé directement aux rayons du soleil pendant un heure précise, c'est à dire, depuis 2h 12' jusques à 3h 12', (le 16 Juillet 1774,) étoit monté à 21°, (79° Fahr.,) et un autre thermomètre, à boule nue, exposé en plein air aux rayons du soleil, à 4 pieds au-dessus du gazon, ne se sotttenoit quà 5 degrés (43° Fahr.)—le lendemain, de retour à Courmayeur, où j'eus le bonheur d'avoir un tems clair, parfaitement semblable à celui de la veille, je choisis un prairie découverte dans laquelle j'établis mon appareil: le thermomètre placé sur le liège noirci montât dans un heure précise à 27°, (93° Fahr.,) et celui qui étoit en plein air à 19°, (75 Fahr.) Tome II. p. 365.

The same accurate observer also found, by comparative trials, that the chemical energy of the solar

Q

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light, as well as its heating power, was much greater upon the summit of the Col du Géant, than on the plain of Geneva.

From these facts, then, I conclude, that the power of solar radiation in the atmosphere, increases from the equator to the poles, and from below, upwards. The obstruction, which the air offers to the passage of the rays, is not alone dependant upon its density at the surface of the earth, for most of the experiments, which establish the difference between the lower and the higher latitudes, were made under nearly equal heights of the barometer.

For the same reason, the difference cannot be ascribed to any change in the cooling power of the medium, for MM. Dulong, and Petit, have established, from experiment, that the velocity of cooling, in any gas, where it is solely owing to contact, remains the same, if the density and the temperature of the gas change in such a way that the elasticity remains constant. Part of the difference, upon the summit of the mountain, may be traced to the diminution of elasticity, but no such cause operated (or, if it did, in a degree too small for appreciation) in the experiments upon the plains.

May not the phenomena be owing to the differences in the thicknesses of the strata through which the radiant matter has to pass? The retardation of its passage, at the equator, may be dependant upon the inflation of the atmosphere over that zone, both from the centrifugal force of the motion of rotation, and from the expansion occasioned by the never-failing heat: its acceleration at the poles, to the comparative thinness of the aërial stratum, dependant upon cold, and a state of rest. As we ascend

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in the atmosphere, we obviously diminish the height of the stratum above us, and therefore the effect upon mountains may be referred to the same explanation.

It is well known that a considerable portion of the light of the sun is always detained and absorbed in its passage through the atmosphere; and it has been calculated, that a vertical ray, shot through the clearest air, would lose more than a fifth part of its intensity. This absorption, supposing the aërial covering of the earth to be every where equal, would be in proportion to the obliquity of the rays; for, as their course receded from the perpendicular, they would have to encounter a greater thickness of the aërial fluid. But if the form of the atmosphere be that of a greatly-oblate spheroid, the thickness of the equatorial stratum may readily be conceived to counterbalance or exceed the obliquity of the course by which the rays would penetrate to the flattened regions of the pole. Now the inequality of the temperature of the earth must evidently impress such a figure upon the elastic atmosphere; and supposing the pressure to be every where equal, and the heat to increase from the pole to the equator, from 0° to 80°, the equatorial axis would be to the polar, as 6 to 5.

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Let Fig. 1 represent a sphere, surrounded by an atmosphere every where equi-distant from the centre; and Fig. 2, the same sphere, with an atmosphere gradually expanding from the poles to the equator. Of the parallel rays, a b, c d, falling upon the surface of the sphere, the power of c d, in the first example, is diminished in proportion to the distance ce to a f; while, in the second example, the distances being equal, the power is undiminished. It may be objected, that the expansion caused by heat, does not, in fact, increase the quantity of matter through which the rays are obliged to pass, although it extends that matter through a larger space; and that, therefore, the proof is wanting of such diffusion increasing the obstruction.

The cooling power, however, of air, as I have before stated, has been proved to be in proportion to its elasticity; and, therefore, it is reasonable to suppose that the difficulty with which heat passes through it, is in the same proportion. The heat of the sun thus sets limits to its own energy; and by an admirable adjustment, the force of radiation is tempered in those regions where its full perpendicular action would be destructive to vegetable existence, and fully developed in climates where its utmost force is required, to counterbalance the ob-

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liquity of its course. If the experimental results could be obtained with sufficient precision, they would form the data of some curious and instructive calculations.

These suggestions, however, are offered with much diffidence, as I am well aware that the subject stands greatly in need of further elucidation. One great object which this arrangement answers, in the economy of nature, is too obvious to be passed over; I mean the additional stimulus which is thus afforded to vegetation in the polar regions, during the short but cheering visit of the sun to their inclement skies. The rapidity with which the earth, when it is uncovered of ice and snow, becomes covered with verdure at the first return of summer, has been often noticed; a rapidity which is totally unequalled at the departure of winter in more temperate climates. Most of the plants spring up, flower, and afford seed in the course of a month or six weeks.

Having traced some of the modifications to which radiant caloric is subject in its passage from the sun to the earth, and having shewn the importance of a further development both of the cause and its effects, I shall now endeavour to collect and combine some particulars with respect to the radiation of heat from the surface of the earth into space; a process, in which the welfare of the vegetable kingdom is no less concerned than in that which we have just been considering. My journal of observations contains a column, also, of the results of experiment upon this division of the subject; they are complete for nearly the whole of the three

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years. They were obtained by exposing, upon short grass, a register thermometer to an open aspect of the sky, having its bulb covered with black wool. The lowest depression, during the night, was entered in the register. The theory and practice of this experiment have been so clearly elucidated in Dr. Wells's Essay, that I can have nothing to add upon the subject: my only aim is to carry a little further the observation of a principle, which, from limited experience, but with a masterly hand, he has shewn to be of such vast importance. The following table exhibits the mean effect of radiation for every month, deduced from the averages of the three years, together with its greatest observed intensity in the same intervals. The first column shews the month, the second the minimum temperature of the air, the third the mean effect, and the fourth the maximum force of radiation:—

TABLE XII. Shewing the Mean Minimum Temperature of the Air, with the Mean and Maximum Force of Terrestrial Radiation for every Month in the Year.

Mean Min. of the Air Mean Depression from Radiation. Max. Depression from Radiation.
January 32.6 3.5 10
February 33.7 4.7 10
March 37.7 5.5 10
April 42.2 6.2 14
May 45.1 4.2 13
June 48.1 5,2 17
July 52.2 3.6 13
August 52.9 5 2 12
September 50.1 5.4 13
October 42.1 4.8 11
November 38.3 3.6 10
December 35.4 3.5 11

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In the last column we may observe an approximation to the law of radiation, established by the experiments of MM. Dulong and Petit; namely, that the velocity of cooling in vacuo (or force of radiation) increases as the terms of a geometrical progression for excess of temperature in arithmetical progression. The power of radiation, as exhibited in the table, has evidently a tendency to increase with the heat, although the effect is masked by too many disturbing causes to have enabled us to discover the law of its progression. The amount of effect denoted in the third column is principally dependant upon the clearness of the atmosphere, and it affords no bad estimate of the comparative brightness of the different months. April appears to be the clearest month of the year, and the cloudy state of July in the midst of summer is very remarkable.

From the particulars of the diary it will be found, that vegetation is liable to be affected at night, from the influence of radiation, by a temperature below the freezing point of water ten months in the year; and that even in the two months, July and August, the only exceptions, the radiant thermometer sometimes falls to 35°.

The comparative experiments of Captain Sabine between the tropics are as follows:—

At Bahia he exposed upon grass to the aspect of the sky, an alcohol thermometer, registering the extreme cold, and having its bulb covered with black wool. The following is the comparison between its indications and those of a register thermometer placed under shelter.

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TABLE XIII. Experiments upon Terrestrial Radiation at Bahia.

Temp, of Air. Temp, of Radiation. Difference. Observations.
July 24 68 63.5 4.5 Dew.
25 68 63.5 4.5 Ditto
26 72 62.5 9.5 Ditto
27 70 61. 9. Ditto
28 64 60.5 3.5
29 67 59.5 7.5
30 65 64 1

The register of cold was the same, whether the thermometer was placed on a grass-plat or on a thick bed of Rotboëlia, or on thick tufts of Poa.

At Jamaica, the radiating thermometer was placed in the manner before described, in contact with the vegetation, and supported by it about ten inches above the ground. The following are the results:—

TABLE XIV. Experiments upon Terrestrial Radiation at Jamaica.

Temp, of Air. Temp, of Radiation. Difference.
October 25 76 72 4
26 76 69 7
27 76 65 11
28 76 66 10
29 76.5 65 11.5
30 76 65 11
November 3 76 67 9

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On the mountains, at 4000 feet above the level of the sea, the thermometer, laid upon grass, afforded the following comparison:

TABLE XV. Experiments upon Terrestrial Radiation upon the Mountains of Jamaica.

Date. Time. Temp. of Air. Temp. of Rad. Differ. coce. OBSERVATIONS.
Oct.31 P.M.10 65 51 14 Clear and calm.
Nov.1 A. M. 5 63 45 18 Ditto ditto
P. M. 11 64 51 13 Clear & gentle
2 A.M.5 64 55 9 Ditto ditto

From all these experiments taken together, it would appear that the same cause which obstructs the passage of radiant heat in the atmosphere from the sun, opposes also its transmission from the earth into space. The force of radiation for the given temperature is less between the tropics, than at the latitude of London; and it obviously increases as we ascend above the surface of the earth.

I have sought, unsuccessfully, for facts which might, tend to throw any light upon the power of terrestrial radiation in the arctic regions; but it is to be hoped that this subject, as well as that of the solar power in the same latitudes, will shortly be determined by the indefatigable activity of Captain Sabine. The intense cold which was found to prevail, during calm weather, in Melville Island, so much beyond the amount of previous calculation, is a strong argument in favour of an increased effect.

In Captain Scoresby's Journal, the following ac-

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count of the freezing of the sea while the temperature of the air was considerably above the point of congelation, must evidently be attributed to radiation of a very powerful degree.

"In cloudy weather, no freezing of the sea, I believe, ever occurs, when the temperature is above 29°; but in clear calm weather, the sea, in the interstices of the ice, generally freezes on the decline of the sun towards the meridian below the pole, though the temperature be 32°, or higher. In the instance now alluded to, the freezing commenced when the temperature was 36°, being 7½° or 8° above the freezing point of sea-water. About 2 A.M. the thermometer in the air fell to 33°, by which time the bay-ice was of such consistence that the headway of the ship, under a light breeze, was sometime stopped by it." Scoresby's Journal, p. 291.

I shall now proceed to detail some further experiments which I have made at different times, the results of which may not prove unimportant to the general subject of radiation. The apparatus which I employed was a concave reflector of copper, plated with silver, of a parabolic form; its diameter was 6 inches, and the length of its focus l¼ inches. Through a collar in its side a thermometer could be passed, and its bulb fixed in the focus, the scale being kept on the outside. This reflector was placed upon a foot with a ball and socket joint, that enabled it to turn in any direction.

Dr. Wollaston was the first to expose a concave metallic mirror, turned upwards to the free air, with a thermometer placed in its focus; and proved the lowering of its temperature after its being thus ex-

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posed for a short time. This experiment he made before the publication of Dr. Wells's Essay, but it does not appear that he pursued the subject further. Mr. Leslie, some years afterwards, adapted the differential thermometer to this idea, and contrived an instrument which he has called an Æthrioscopc. This was nothing more than a metallic reflector, with one of the balls of a differential thermometer placed in the focus, and the other out of it. He confirmed Dr. Wollaston's result, and the thermometer fell, upon being exposed to a clear sky. The effect he found to depend upon the clearness of the atmosphere.

Mr. Leslie, in his description of the construction and uses of his Æthrioscope, has, unfortunately I think, indulged in a brilliancy of imagination and figurativeness of language, which have greatly obscured his meaning. He ascribes, for instance, the action of the instrument to " cold pulses showered entire from the heavens." He speaks of " the higher strata of the atmosphere darting cold pulses downwards, and the lower strata projecting equal pulses of heat upwards."

The Æthrioscope, he says, " extends its sensation through indefinite space, and reveals the condition of the remotest atmosphere." Nay, more, he expects that " when constructed with greater delicacy, it may, perhaps, scent the distant winds, and detect the actual temperature of different portions of the heavens." With far humbler views I have made considerable use of Dr. Wollaston's apparatus, which, for reasons which I shall not now stop to discuss, I very much prefer to Mr. Leslie's. The

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standard thermometer, to which all my observations refer, when not otherwise expressed, had its bulb covered with black wool.

My first object was to ascertain the force of radiation from a thermometer, so guarded from the influence of surrounding bodies, compared with another, exposed, as I have before described, upon grass. The following Table exhibits the results. The first column shews the lowest temperature of the air during the night, the second the lowest temperature of a thermometer on grass, and the third that of the thermometer in the reflector.

TABLE XVI. Comparison of the Force of Radiation in a Reflector and on the Grass.

Temp. of Air. Temp. of Grass. Temp. in Reflector. OBSERVATIONS.
42 34 30 Very fine and clear.
47 39 35 Ditto ditto.
52 44 42 Ditto ditto.
44 35 32 Ditto ditto.
44 36 34 Ditto ditto.
54 48 45 Ditto ditto.
58 52 52 Dull.
57 51 49 Very fine–moon hazy.
56 51 50 Light clouds.
51 41 41 Very fine and clear.
45 35 35 Ditto ditto.
50 42 41 Ditto ditto.
50 42.3 40.5

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The average difference is not quite two degrees.

I consider this as much the most accurate method of measuring the force of terrestrial radiation: at the same time it is gratifying to find that the means which I had adopted, before this idea had occurred to me, were not, upon a mean of observations liable to any very considerable error. The radiant thermometer is so completely insulated by the reflector, from the counter-radiation of surrounding bodies, that it may be applied with equal effect in any situation where the aspect of the sky is very limited. Even in the streets of London, where the radiation of an exposed thermometer is nearly neutralized, and the utmost effect never exceeds two or three degrees, that of the thermometer, guarded by the reflector, is wholly unimpeded. Experiments that are thus made, in whatever situation, are strictly comparable, provided they are screened from any strong action of the wind.

Being thus in possession of the means of cutting off the access of radiant matter from any source, and of directing it to any required object, I was anxious to ascertain the force with which it was given off while the sun was above the horizon, compared with what it was in the absence of that luminary. Under the most favourable circumstances, when the air was calm and the atmosphere clear, I never could obtain an effect of more than five or six degrees with the thermometer covered with black wool. It then occurred to me, to try the influence of colour in modifying the results. I had another reflector made exactly similar to the former, and their power, upon trial, was found to be precisely

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equal; that is to say, the radiating thermometer fell to an equal amount in each of them. I now covered the ball of a thermometer with white wool, and placed it in the focus of one reflector, and the thermometer with black wool in the focus of the other. I selected a cloudless day for the experiment, and placed the two instruments, side by side, in the shade of a tree, inclining them at equal angles towards the clear eastern sky. The following Table includes the results.

TABLE XVlI. Comparison of the Force of Radiation from Black and White Wool.

May 10. Radiation from Black Wool. Radiation from White Wool. Tamp. or Air. OBSERVATIONS.
P.M. 3½ 58 53 63 Atmos. cloudless.
4 58 53 63 During the experiment the reflectors were changed.
8 44 43 54
11 36 36 47
During Night 35 35 45

The amount of radiation, therefore, from the white wool, was equal, during the time the sun was high in the heavens, to what it was during the night; while it was one-half less from the black wool. During the absence of the sun, the radiating power of the two was equal.

The experiment was repeated under varying circumstances; and in examining the results, as included in the following Table, it will be necessary to attend particularly to the collateral circumstances.

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TABLE XVIII. Comparison of the Force of Radiation in Black and White Wool.

May 17. Radiation from Black Wool. Radiation from White Wool. Temp of Air. OBSERVATIONS.
P.M.1½ 64 59 65 Overcast, with cumulo-stratus.
2 68 60 65 Clearing–reflectors turned to clearing space.
73 62 65 Faint sun-snine.
74 62 66 Strong aun-shine–reflectors turned, so that the shadows of the bulbs just appeared on the metal.
4 76 64 68 Lightly overcast.
Night. 51 51 55 Fine.

The power of radiation was nearly neutralized in the black wool, while the sky was over-cast, but in the white wool was only reduced to about one-half. As the sky cleared, the reflectors being turned towards the sun's place, the black thermometer rose above the temperature of the air, and the white thermometer still gave off more heat than it received. In full sun-shine, the reflectors being just turned out of the direct rays, the black thermometer rose 8° above the temperature of the air, and the white thermometer fell 4° degrees below it. In estimating these effects it must be remembered, that the action of the reflector, in receiving and transmitting heat, is different. In the former case, we have an exaggerated action; the heat which falls upon the surface of the speculum is thrown upon the thermometer in a concentrated form. In the latter case, the heat, which radiates from the

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thermometer in the focus, falls upon the concave metal, and is reflected into space in parallel lines. The effect is, therefore, only slightly augmented from the larger aspect of the sky. Whenever the reflector, with the black-wooled thermometer, is turned, while the sun is above the horizon, towards a cloud, the mercury rises above the temperature of the air, excepting in the winter months; and a distinct effect is produced even from the quarter most distant from the sun. The concrete vapour seems to disperse the radiant matter, and to act upon it in much the same way as ground glass upon transmitted light. I subjoin some experiments which illustrate this point, and shew the effect of two similar thermometers in similar reflectors, directed to different quarters of the heavens.

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TABLE XIX. Effects of Radiation under different Aspects of the Sky.

Date. Hour. Position of the Reflector. Temp. of Black Wool. Temp. of Air. OBKRVATIONS.
July 2 12 Horizontal
Inclined 30°
76
79
63 Sky overcast—cumulo. stratus—sun's place not visible—and brisk wiod from S.W.
1 Horizontal
Inclined 30°
83
83
63 Sun's place just visible, but no shadows.
2 Horizontal
Inclined 30°
86
86
63 Ditto ditto
2 ½ Horizontal
Inclined 30°
83
82
63 Sun's place not visible.
3 Horizontal
Inclined 30°
79
79
63 Ditto ditto
Inclined N. 30°
S. 30°
76
79
63 Ditto ditto
Inclined N.E. 30°
S.W. 30°
85
88
63 Ditto ditto
3 ½ Inclined N.E. 30°
S.W 30°
96
100
63
Inclined W.
E.
55
55
61 Just before sun-set.
11 P.M. Inclined N.
S.
57
57
61
Night. Inclined N.
E.
40
40
48 Fine.
July 4 All Night. Horizontal
Vertical
41
43
51 Very fine.

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No effect is produced by any cloud after the sun has sunk below the horizon; and in an overcast night the action of radiation is perfectly neutralized. I have appended to this essay a series of observations made in London, at different hours of the night and day, from January to April, 1822.

As the power in different bodies of absorbing heat, and the power of emitting it, do not appear to be equal under every circumstance, as has been demonstrated in the case of the black and the white wool, it becomes a curious inquiry to ascertain the relation of various substances to these effects. I regret that I have not had leisure to pursue this branch of the subject with the attention which it deserves. I shall subjoin the results of two or three experiments, to shew that much curious information might be expected from the investigation. The standard of comparison was, in all cases, the black-wooled thermometer, and the substance compared was stuck upon a thermometer, in a similar reflector, by its side.

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Substance Compered. Radiation. Black Wool. Temperature of Air. OBSERVATIONS.
May 21 11 P.M. Naked Alcohol Therm. 48 46 55 Very fine night.
Night. 40 38 48
22 9 A.M. 53 58 56 Haze.
23 8.P.M Garden Mould. 41 38 46 Very fine.
11 40 37 46 Ditto
Night. 38 35 45 Ditto
24 9 P.M. 59 59 56 Ditto
8 P.M Chalk 46 43 53 Ditto
11 43 40 50 Ditto
Night. 35 33 44 Ditto
25 A.M. 11 67 61 62 Lightly overcast.
18 P.M. 11 Leaf of the Rose-Campion. 46 45 57 Very fine.
19 P.M. 8 51 50 61 Ditto
11 44 48 53 Ditto
Night. 39 38 48 Ditto
22 P.M. 11 Integuument of the Flower of an Iris. 44 42 52 Ditto
Night. 38 37 47 Ditto

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Whilst engaged in this course of experiment, it occurred to me that a favourable opportunity presented itself of determining a question which has at different times occasioned considerable controversy, and concerning which, many discordant statements have often been made: I mean, the radiation of heat from the body of the moon. Dr. Howard has lately published the following result of an experiment by means of a delicate differential thermometer, which seems to establish the reality of such an effect.

"Having blackened the upper ball of my differential thermometer, I placed it in the focus of a thirteen-inch reflecting mirror, which was opposed to the light of a bright full-moon. The liquid began immediately to sink, and in half a minute was depressed 8°, where it became stationary. On placing a screen between the mirror and the moon, it rose again to the same level, and was again depressed on removing this obstacle."—Sillimar's Journal, vol. ii. p. 329.

Upon reading the above extract, it struck me that it did not clearly explain in which leg of the instrument the depression of the liquid took place; and that the effect, as described, might just as well be attributed to the radiation of heat from the blackened bail of the thermometer, as to radiation to it from the moon. To determine this doubt, I tried the following experiments:—

I selected an unexceptionable opportunity, 26th of December, 1822. The moon was in that part of her orbit when she is nearest to the earth, and was approaching to the full. The atmosphere

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was cloudless, and perfectly calm. The smallest writing was distinctly legible in the moon-light. At 9 P.M. the temperature of the air was 28°. I placed the black thermometer in the focus of the reflector, and directed it to a part of the sky at a distance from the moon. In a few minutes it fell to 20°, and was stationary. I then turned it immediately towards the moon, and caused the focus of light to fall upon the ball of the thermometer. It still remained stationary at 20°, and for half an hour, during which the rays were concentrated upon it, the mercury never moved.

At 11 P.M. the temperature of the air 27°
reflector turned from the moon 19°
— in the moon-beams 19°
Dec. 28th, 7 P.M.
Moon full—atmosphere perfectly calm and clear.
Temperature of the air 24°
Reflector turned from the moon 15°
— in the moon-beams 15°
At 11 P.M. the sky became lightly clouded, and the amount of radiation was only 2°.
Temperature of air 22°
Radiating thermometer 20°

Thus it appears that, so far from possessing the power of radiating heat to the surface of the earth, the moon does not even diminish the amount of radiation from the earth; and the lightest vapour is more efficacious in this respect than the concentrated influence of the lunar light.

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Observations of a Black Radiating Thermometer in a Concave Reflector.

Turned to the North, Angle 30°.

Date Hour. Temp of the Air. Day. Night Difference state of Reflector. State of Weather.
1822 Jan. 13 p.m 4 48 45 -3 Fine, but misty
11 45 37 -8 Ditto ditto
night 40 32 -8 Very fine
14 a.m. 10 42 35 -7 Sports of rain. Very fine and clear
p.m.4 42 35 -7 Cloudless
11 39 30 -9 Light clouds
night 40 36 -4 Ditto ditto
15 a.m.9 40 32 -8 Bright Very clear
p.m.11 31 21 -10 Ditto ditto
night 30 20 -10 Ditto ditto
16 a.m. 9 31 22 -9 Ditto ditto
p.m.4 31 21 -10 Ditto ditto
11 29 19 -10 Ditto ditto
night 29 19 -10 Ditto ditto
17 a.m.9 33 33 0 snow
p.m.11 33 26 -7 Very fine
night 32 25 -7 Ditto ditto
18 a.m. 9 35 30 -5 Bright. Ditto ditto misty
p.m. 4 39 31 -5 Ditto ditto ditto
11 39 37 -2 Overcast and dull
night 38 33 5 Dull
19 a.m.9 41 41 0 Tarnished. Dull and fogyy
26 a.m. 9 41 41 -8 Vary fine
p.m.11 40 35 -5 Dull
night 32 22 -10 Dull and spotted Very fine
27 a.m. 10 34 27 -7 Diito ditto
p.m. 4 47 47 0 Overcast and dull
11 40 40 0 Ditto ditto
night 35 33 -2 Dull
28 a.m 9 45 43 -2 Spotted with rain Mild and misty
p.m. 11 41 39 -2 Overcast and dull
night 35 27 -8 Very fine but misty
29 a.m. 9 37 31 -6 Blacks in the mirror Ditto ditto
p.m.11 38 34 -4 Lightly overcast
night 32 22 -10 Very fine
30 a.m. 9 34 26 -8 Ditto dittu
P.M 11 34 30 –4 Fog

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Date Hour. Temp of the Air. Day. Night Difference state of Reflector. State of Weather.
night 30 23 -7 Fine
31 a.m. 9 35 28 -2 Dew upon lower half Light clouds
p.m.11 35 28 -7 Drops of rain
night 35 28 -7 Fine
1 a.m. 9 42 37 -5 Spotted with rain Very fine
p.m.11 38 29 -9 Ditto ditto
2 a.m. 9 47 47 0 Tarnished & spotted Small rain
p.m.11 49 45 -4 Fine
night 39 39 0 Full of rain. Rain
3 a.m. 10 44 39 -5 Very fine
p.m. 11 37 28 -9 Ditto ditto
night 33 25 -8 Ditto ditto
4 a.m. 9 38 36 -2 Moisture running off the bulb Foggy
p.m.11 47 45 -2 Overcast
night 38 37 -1 Stormy
5 a.m.10 49 42 -7 Fine
p.m. 11 39 30 -9 Very fine
night 32 22 -10 Hoar-frost upon the bulb and stem Ditto ditto
6 a.m.10 35 27 -8 Ditto ditto
p.m 11½ 41 -35 -6 Tarnished. Ditto ditto
7 a.m.9 45 43 -6 Dull and close
p.m.3 48 43 -3 ditto ditto
11 47 43 -4 Ditto ditto
night 45 41 -4 Full of rain. Ditto ditto
8 a.m.9 48 45 -3 Overcast but fine
p.m. 11 45 42 -3 Ditto ditto
night 42 36 -6 Spotted with rain Ditto ditto
9 a.m.10 47 47 1 0 Overcast and mild
p.m.11 49 47 -2 Ditto ditto
night 46 41 -5 Ditto ditto
10 a.m. 10 47 45 -2 Ditto ditto
p.m.11 47 45 -2 Ditto ditto
night 41 -33 -8 Spotted with rain Very fine
11 a.m. 10 44 41 -3 Ditto ditto
12 p.m.11 39 36 -3 Stained. Fog
night 37 33 -4 Some water. Ditto
13 a.m.9 41 38 -3 Fine but misty
p.m.4 44 38 -6 Ditto ditto
night 40 32 -8 Very fine
24 a.m. 10 46 48 +2 Lightly overcast

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26 -10 Spotted with rain Very fine
-8 Ditto ditto
24 -11 Ditto ditto
22 -9 Hoar frost on bulb Ditto ditto
-4 Ditto ditto, fog
-6 Very fine
26 -9 Ditto ditto
20 -10 Hoar frost on bulb Ditto ditto
33 -6 Light clouds
31 -3 Ditto ditto
+1 Overcast and mild
36 -7 Very fine
+4 Turned to a dente cloud
-4 Very fine
28 -9 Ditto ditto
+6 Overcast and dull
+5 Lightly overcast
-2 Ditto ditto
51 -3 Ditto ditto
46 -5 Fine
+4 Lightly overcast
-6 Very fine
37 -9 Ditto ditto
+1 Overcast
-5 Very fine
49 -2 Overcast
31 -10 Very fine
+5 Ditto ditto
-7 Very fine
37 -9 Ditto ditto
+2 Dense clouds

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Date Hour. Temp of the Air. Day. Night Difference state of Reflector. State of Weather.
1 p.m.5 46 46 0 Overcast and dull
11 42 32 -10 Very fine
11½ 43 39 -4 Lightly overcast
night 39 31 -8 Fine
3 a.m.10 50 59 +9 Lightly overcast
p.m.1 54 60 +6 Ditto ditto
2 53 61 +8 Ditto ditto
3 53 50 -3 Clear
5 50 42 -8 Very fine
11 43 35 -8 Ditto ditto
night 40 31 -9 Ditto ditto
4 a.m. 10 49 58 +9 Lightly overcast
10½ 51 61 +10 Ditto ditto
11 51 62 +11 Ditto ditto
12 51 60 +9 Ditto, drops of rain
p.m.½ 52 64 +12 Ditto ditto
1 52 58 +6 Ditto ditto
51 53 +2 Clearing
5 49 47 -2 Ditto
11 47 42 -5 Overcast
night 43 34 -9 Fine
5 a.m.10 51 62 +11 Lightly overcast
p.m.5 51 50 -1 Ditto ditto
11 46 37 -9 Very fine
night 43 33 -10 Ditto ditto
6 a.m.10 52 52 0 Ditto ditto
p.m.5 49 49 0 Ditto ditto
11 41 31 -10 Ditto ditto
night 36 27 -9 Ditto ditto
7 a.m.10 45 51 +6 Overcast
night 36 27 -9 Very fine
8 a.m.10 45 46 +1 Ditto ditto
p.m.5 39 39 0 Hail showers
night 35 30 -5 Fine
15 a.m.10 57 67 +10 Lightly overcast
16 p.m.5 54 53 -1 Dull
11 46 37 -9 Very fine
night 41 34 -7 Foggy
28 a.m.10 56 67 +11 Very hazy
night 46 40 -6 Fine

[page 250]

[page 251]

AN ESSAY

UPON THE

HORARY OSCILLATIONS OF THE BAROMETER.

THE barometric column was first observed to have a daily periodical vibration between the tropics, by the expedition under the command of the unfortunate Peyrouse. M. Lamanon, the naturalist, has given an account of these observations in the second volume of the voyage, at page 521. He states that from about the 11th degree of north latitude, he began to perceive a certain regularity of motion in the barometer, so that the mercury stood highest about the middle of the day, from which time it descended till the evening, and rose again during the night. As they approached the equator, the effect became more distinct, and on the 28th September (1785) a series of Experiments were begun in I° 17′ north latitude, and continued for every hour, till the 1st of October. The following are the results of the observations on the 28th and 29th.

Sept. 28. From 4 to 10 A.M., barometer rose 0.19 inch.
From 10 A.M., to 4 P.M. fell 0.12
From 4 to 10 P.M. rose 0.09
Sept. 29. From 10 (28th) to 4 P.M. fell 0.13
From 4 to 10 A.M. rose 0.15
From 10 A.M. to 4 P.M. fell 0.13
From 4 to 10 P.M. rose 0.10

The observations on the 30th were to the same effect.

Hence it was inferred that there is a periodical flux and reflux of the atmosphere, at the equator,

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producing in the barometer a variation of about 0.12 inch (English,) corresponding according to M. Lamanon to a height in the atmosphere of about 100 feet. The latitude of the ship on the 28th, was 0° 50' north; and 0° 11' north, on the 29th.

In the year 1794, Dr. Balfour published, in the Asiatic Researches, an account of some observations made at Calcutta, which agreed in a remarkable manner in the same conclusion. During one whole month, he observed the barometer every half hour: the mercury constantly fell from ten at night to six in the morning; from six to ten in the morning it rose; from ten in the morning, to six at night, it fell again; and, lastly, rose from six to ten, at night. The maximum height was therefore at ten, P. M., and ten, A. M., and the minimum at six, P. M., and six, A. M. The oscillations sometimes amounted to 0.1 inch, but in general were considerably less.

The observations of M. de Humboldt, of a later date, confirm the existence of these semi-diurnal variations in the torrid zone, and extend them to the south of the equator. According to his results, the barometer generally falls from ten o'clock, A.M., till 4, P.M.; then rises again till ten, P.M. —again drops till four, A.M., and mounts till ten, A.M.

Captain Sabine, also, amongst his other numerous, laborious, and interesting, pursuits, turned his attention to this subject, while between the tropics, and has favoured me with the following results of his Experiments, to ascertain the amount of the horary oscillation at Sierra-Leone, St. Thomas', Trinidad, and Jamaica:—

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[page] 253

It is not, however, alone-in tropical latitudes, that these horary motions of the barometer may be detected: the absence of those disturbing causes which affect the atmosphere in temperate climates, and produce the much more considerable but irregular fluctuations of the mercurial column, render them more prominent in those situations; but by a system of averages, which balances the irregularities, the regular movement is elicited, even when most concealed, M. Ramond found at Clermont-Ferrand, in latitude 45° 47", that a mean often days sufficiently neutralized the irregular oscillations, and the periodical motions were distinctly exhibited in intervals of that length. The hours of the fluctuations were, as nearly as possible, coincident with those at the equator; but the effect was considerably less, and did not amount to more than 0.039 inch. The monthly means of the observations made at the observatory at Paris, present the same result, with a still further reduction of the effect; the average of six years' observations being .028 inch: and, lastly, my own meteorological journal exhibits the horary movements with great regularity, but only to the average extent of 0.015 inch.

Thus, there can be no doubt that the suggestion of Captain Sabine is correct, that "the amount of the atmospherical tides diminishes progressively from the equator to the tropics," and, further, that it continues at a diminishing rate, at least as far as the fifty-second degree of latitude. The following Table presents a synoptic view of the mean results.

[page] 254

TABLE V. Shewing the Mean Periodical Movement of the Barometer at different Latitudes.

Names of Places. North Latitude. Mean Periodical Movement of the Barometer.
St. Thomas' 0° 24' 0.074 Inch.
Sierra Leone 8° 29' 0.073
Trinidad 10° 39' 0.063
Jamaica 17° 56' 0.058
Clermont-Ferrand 45° 47' 0.039
Paris 48° 50' 0.028
London 51.31 0.015

There can no longer, therefore, be any hesitation in admitting that, while the irregular movements of the atmosphere and the general range of the barometer increase, in going from the equator towards the poles, there is a regular concomitant fluctuation, which augments, as we proceed from high latitudes towards the equator. This phenomenon presents an universally acknowledged difficulty, and is, as yet, one of the unresolved problems of Meteorological Science. Attempts have been made, by some, to explain it upon the supposition of a tide produced as in the waters of the sea; but the regularity of its horary recurrence is obviously inconsistent with the notion of lunar influence. Others have imagined it to be dependant upon the alternations of land and sea breezes; but observations made in mid-ocean, totally disprove the fact.

I must solicit indulgence for the following attempt to solve the difficulty.

[page] 255

In drawing up my essay upon the constitution of the atmosphere, this, amongst the other phenomena, occupied much of my attention: but I have only slightly alluded to it amongst the meteorological facts in the third part. I thought it better, as it presented matter of some debate, and as its explanation was not essentially necessary to the general investigation, to reserve its consideration to a separate paper. Those who have had the patience to peruse my first pages, will, probably, most readily comprehend the following ideas.

Let us suppose that in the atmosphere surrounding the earth, a circulation is kept up between the poles and the equator; and that the cold dense air of the former regions flows in a lower current to the latter, while the elastic air of the latter is returned fai an upper current to the former. There can be no difficulty in imagining further that, as long as these currents are maintained with regular velocities, a barometer, at all intermediate stations, might exhibit an equal pressure of the aërial columns: for as much air would flow from their summits, as would be returned to their bases. A general alteration of temperature which equally pervaded both currents, would produce no alteration in the weight of a vertical section, comprising both; nor would a partial alteration, equally diffused through the upper and under sections of any one column. The velocities of the currents would be partially altered, but the higher and the lower would still compensate each other. But an alteration of temperature, which affected the upper and lower currents unequally, would produce partial expansions and contractions,

[page] 256

which would effect an unequal distribution of the ponderable matter. If the lower stratum of any perpendicular section were expanded by heat, while the upper were unaffected, the outgoing current of that section would be increased, while the in-coming current would be checked; and the balance of the two being disturbed, the total weight would be diminished: and, on the other hand, a local decrease of temperature would produce the analogous contrary effect. Now, the alternations of heat and cold, produced by the changes of day and night, may be regarded, in a general way, as equally affecting both the main currents of the atmosphere, and as equally pervading the whole length of the aërial columns. The heating surface being below, the warm particles quickly ascend, and are immediately replaced by the colder particles from above; and, by this vertical circulation, the diffusion of the heat is very rapid. But a minuter examination will satisfy us that, though rapid, this action is not in effect instantaneous; and the lower stratum, which is in contact with the heating surface, must, in the act of receiving heat, have its temperature disproportionately augmented.

The exchange of particles between the upper and lower strata, must occupy some time, however small the interval: the consequence must be, that the barometer will measure by its fall the amount of this inequality. So, on the other hand, in the process of cooling, in the absence of the sun, experiment has proved, that the lower strata of the air become more rapidly affected by radiation than the upper, and the total increase of weight from this

[page] 257

cause will be shewn by the rise of the mercurial column.

Let us endeavour to trace this effect a little more minutely along any given meridian, beginning at the equator.

At this station, the only circumstance which we have to appreciate is, the irregularity of the lateral expansion or contraction. As the earth acquires warmth from the sun, the barometer falls; but the check thus communicated to the in-coming currents from the poles, must be felt along the whole line of their course; and their due velocity being opposed, without any adequate compensation in the upper currents, the barometer, from this cause, would have a tendency to rise at all latitudes between the equator and the pole. Assuming then an intermediate station upon the same meridian, we should have the same effect produced by the unequal expansion of the lower current of the atmosphere, but opposed now by the impulse communicated from the equator. The fall of the barometer would only then represent the balance of the two effects, and must be less than at the equator. The farther we proceed towards the pole, the more must the revulsive action accumulate, and the less must the balance of the two become, till, at some neutral point, they are exactly equal. Beyond this point, again, the former action would exceed the latter, and the barometer would rise in the higher latitudes while it was falling in the lower.

The following Figure may possibly tend to illustrate these propositions.

S

[page] 258

Let the parallelogram a b e d represent the lower current of air flowing from the pole to the equator in its undisturbed state, and the perpendiculars h i j k, &c., different degrees of latitude. Let the lesser parallelogram a b e f be the equal diminution of weight which would arise from the partial expansion by the increase of daily temperature, and the triangle e g f the gradually increasing density arising from the retardation of the current. The rise and fall of the barometer on either side of the neutral point k would then be represented by the portions of the perpendiculars included between the hypothenuse e g and the side of the parallelogram a b.

The results of observations in different latitudes, included in the preceding Table, obviously coincide with such a gradual progress towards a neutral point; but we have as yet no experiments to prove the corresponding opposite effect beyond this limit. Whilst considering this subject, it occurred to me, that Captain Parry's observations at Melville Island, might possibly afford some light upon this

[page] 259

interesting point. Upon consulting, however, the meteorological register, as published in his Journal, I was disappointed to find, that it only recorded the maximum and minimum height of the barometer in the twenty-four hours without mentioning the periods of their recurrence. I happened, very fortunately, to discourse with Captain Sabine upon the subject, and he assured me, that the observations were made and entered four or six times a day with the utmost regularity, and very obligingly offered to apply to the Admiralty for liberty to inspect the manuscript. His application was immediately complied with, and I was fevoured with the loan of the original registers.

I found, upon inspection, that the Journal had been kept with the greatest precision, and the height of the barometer had been entered, during part of the time, at four regular periods, viz., 6 A.M., noon, 6 P.M., and midnight; and the remainder of the time six times in the twenty-four hours, viz., 4 A.M.,8 A.M., noon, 4 P.M., 8 P.M., and midnight. I immediately, with the utmost interest, undertook the arrangement of the observations to suit my purpose. I selected the twelvemonth from Sept. 1819, to August 1820, during which time the Hecla was constantly between latitudes 74° and 75°, and the greater part frozen up in Winter Harbour. The following Tables exhibit the results of my calculations. The first contains the monthly mean heights of the barometer and thermometer, taken four times in the day from September to February and part of March, and the second the monthly means taken six times in the day, from the latter part of March to August inclusive.

S 2

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These Tables present a complete confirmation of the opinion which I had formed from theory.

In the first, including the winter half year, it will be observed, that the mean temperature scarcely varied between noon and midnight, the effect of the remote equatorial expansion was therefore unopposed, and the barometer constantly rose from 6 A.M. to 6 P.M., in coincidence with the fall in the lower latitudes. From 6 P.M. to 6 A.M. it as regularly fell.

In the second half of the year, while the sun was above the horizon, the daily variations of temperature were considerable, and the effect less regular; but, nevertheless, the barometer constantly rose from noon to 8 P.M., and then descended to midnight.

I am enabled, by the publication of the account of the Expedition to the Rocky Mountains of America, under the command of Major Stephen Long, to subjoin from their Meteorological Journal, for the same year, a comparison of the motions of the barometer at three different periods of the day, almost upon the same meridian, and at a distance of 33° of latitude. The expedition took up their winter quarters at "Engineer Cantonment," in latitude 41°25′ N., and longitude 95°43′ W., almost the centre of the great North American Continent, and the following Table contains my calculations of the means of the observations made during their stay in this situation.

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TABLE V Hours of the Day at Melville Island.

1819 Midnight
September Temperature
+21.3
-29.890
October -5. -29.825
November -21.2 -29.937
December -21.6 -29.893
January -30.4 -30.063
February -33.5 -29.771
March -20.5 -29.571
208.950
29.8500 Max. 6 P.M. 29.8644
-.0144 Min. 6 A.M. 29.8288
Difference .0356

TABLE VI Hours of the Day at Melville Island.

1819 4 A Midnight.
March Temperature
-29
+29.910
April -29-12.8 -29.987
May +30+13.1 30.109
June +29+33.6 -29.817
July +29+39.1 -29.660
August -29.+30.5 -29.735
179 179.218
29 29.8696 Max.8 P.M 29.8708
- - .0012 Min. noon 29.8631
Difference .0077

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[page] 261

TABLE VIII. Shewing the mean Heights of the Barometer and Thermometer at three different Hours of the Day Rocky Mountains of North America. Lat. 41°.25', long.95°.43'.

Morning Noon Evening
Temp. Barometer. Temp. Barometer. Temp. Barometer.
1819.
September 60. +28.650 80.3 –28.634 72.1 –28.633 Maximum, Morn. 28.713
Minimum, P.M. 28.609
Difference .104
October 40.3 +28.812 61.8 –28.730 55.6 –28.720
November 35.8 +28.705 48. –28.607 46. –28.604
December 20.2 +28.808 29. –28 660 26.3 +28.703
1820.
January
2.1 +28.966 16.9 28.966 10.8 –28.954
February 22.1 +28.618 36.5 –28.501 32. +28.550
March 27.4 +28.902 41.8 –28.815 37. +28.881
April 45.7 +28.465 65.4 –28.267 62.4 –28.261
May 55.2 +28.496 69.7 –28.309 65.8 +28.370
Mean 28.713 28 609 28.630

Notwithstanding the height of this latter station above the sea, we still find the same principle prevail, and it is satisfactory to discover amongst so many various circumstances, whose influence upon the results are at present unknown, that, in accordance with the theory, on the same hours of the same

[page] 262

months, the barometer, upon nearly the same meridian, periodically rose in latitude 74°47′, and fell in latitude 41°25′.

It must be acknowledged that much remains to be done for the complete elucidation of this subject.—Much, that is not difficult of performance, but requires extensive co-operation, and some nicety of observation. Had the numerous meteorological registers, which have hitherto been published, been kept with that exactness and that attention to the accuracy of the instruments employed, which is so necessary in scientific pursuits, the data for these and for other highly important calculations would have been already abundant: but, notwithstanding the multitude of labourers employed in this interesting field, there is a want of unity, and especially a carelessness, in their exertions which render them totally unavailing to the nicer purposes of the science. Let the meteorologist inquire by what means it is, that astronomy, the sublimest of all the sciences, has attained to its present wonderful state of perfection; and he will find that it is by the most microscopic attention to the perfecting of its instruments of research, and by the most faithful precision of unremitting observations; and let him be assured that it is only by the same painful care to minutiœ that his own favourite science will ever be raised to that standard of exactness of which there can be no doubt that it is susceptible, but from which it is to be lamented that it is at present so far removed.

[page 263]

AN ESSAY

UPON THE

CLIMATE OF LONDON.

AFTER the very interesting and laborious work of Mr. Howard, upon the climate of London, it may, at first sight, appear presumptuous in me to claim attention upon a subject which has been so ably and so extensively pre-occupied: but when it is considered that that able philosopher was unprovided with any sufficient means of measuring the quantity, or estimating the changes of the sea of vapour which necessarily permeates and pervades every part of the great aërial firmament; and when it is borne in mind, that one of the main springs of all the wonderful motions of the air, and the changes of the weather, is the slow and silent influence of the aqueous steam; I shall be excused for attempting to elucidate, from experiments, a part of the subject so important, but, hitherto, so neglected.

Its connexion with the vegetable kingdom, and with all the most important processes of the agriculturist, must be evident to the most superficial observer; and it is more than probable, that it will be found of equal importance to those who make a study of the complicated processes of the animal economy.

It has ever been a favourite speculation with phi-

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losophers to trace in the constitution of the atmosphere the origin of some of the diseases which affect the human race. The discovery of pneumatic chemistry, and the new means of questioning nature, which it put into their hands, seemed at first to promise a solution of this interesting problem; and hopes were entertained that the cause of epidemic and local complaints might be found in the varying elements of the compound air we breathe. The eudiometric processes which were immediately instituted and repeated in every part of the world, proved, however, the unvarying proportions of the permanent gases of which it is composed. It is not, therefore, irrational to suppose that an accurate method of estimating the varying quantity of aqueous vapour in the elastic medium which surrounds us, which is the only fluctuating ingredientof its composition, may lead to some useful hints upon this important subject. Certain it is, that some indications of this kind may be perceived even by the healthy, and those who are not conversant with the progress of disease. There are days on which even the most robust feel an oppression and languor, which are commonly and justly attributed to the weather; while on others they experience exhilaration of spirits, and an accession of muscular energy. The oppressive effect of close weather and sultry days, may probably be accounted for from the obstruction of the insensible perspiration of the body, which is prevented from exhaling itself into the atmosphere, already surcharged with moisture; while unimpeded transpiration from the pores, when the air is more tree from aqueous vapour, adds new energy to all

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the vital functions. In bodies, debilitated by disease, indeed, the contrary effects may be produced: they may be unable, from weakness, to support the drain of free exhalation which is exhilarating to the healthy; and hence, probably arises the benefit of warm sea-breezes in cases of consumption, and diseases of the lungs. Observations upon climate, with a more particular regard to the hygrometric stale of the atmosphere, may reasonably be expected, amongst otter certain advantages, to throw some light upon the treatment of these complaints; and may, perhaps, teach us to construct an artificial atmosphere, of greater efficacy than any that has yet been recommended, in cases when the relief of local change may be impossible.

The foundation of the following attempt is a series of observations unremittingly continued three times in the day for three years; a period of time which, though it may not entitle me to say that it includes all the changes occasioned by the revolution of the seasons, is sufficiently long to furnish, by the system of averages, a very near approximation to the distinguishing characters of the climate. I have assumed the data as furnished by my own experience, and it is gratifying to find that, with the barometer and thermometer, my results agree very closely with those of Mr. Howard, who has founded his calculations upon a long series of years, and to whose conclusions, in cases of discrepancy, I willingly cede the preference which is due. The general accordance between us, however, upon these points, encourage me to presume that the indica-

[page] 266

tions of the hygrometer will not be found to differ very far from the real mean.

I shall proceed, first, to consider the general characters of the climate, as derivable from the averages of the three years together; and I shall then endeavour to institute a comparison of the separate years with the mean, and with each other.

I shall not, now, enter into any detailed account of the instruments employed, or the mode of placing and observing them; I shall reserve for another place, a few observations upon the precautions which ought to be taken in making meteorological observations, and shall only here assume credit for moderate care in these particulars. I must, however, premise, that the times of the day, denoted by morning, afternoon, and night, were, for the first, from eight to ten o'clock, A.M.; for the second, from half-past three to half-past five, P.M.; and, for the third, from ten to half-past eleven, P.M.

The mean pressure of the total atmosphere, denoted by the barometer, I find to be 29.881 inches. The mean of twenty years, deduced by Mr. Howard from the observations of the Royal Society, is 29.8655 inches. The mean temperature derived from the daily maxima and minima of the thermometer, is 49.5°, which corresponds even to the decimal place, with Mr. Howard's estimate. The mean dew point I consider 44.5°, it being also calculated from the daily maxima and minima. The elastic force of the vapour is, therefore, 0.334 inch, and a cubic foot of the air contains 3.789 grains of moisture. The degree of dryness is represented by 5° upon

[page] 267

the thennometric scale, and the degree of moisture by 850, upon the hygrometric scale. The average quantity of rain is 22.199 inches, and the amount of evaporation, calculated from the hygrometer, 23.974 inches. The weight of water, raised from a circular surface of six inches diameter, is 0.31 grain per minute.

The accordance of this method of estimating the amount of evaporation with the results of actual measurement is gratifying, and proves most incontestibly the accuracy of the calculations upon which it is founded. From the mean of four years, the gauge being upon, or near, the ground, Mr. Howard found that the annual results averaged 21.46 inches. Of these, one year, the summer of which was hot and dry, afforded 25. inches, and when it is considered that two out of the three summers now in question were likewise distinguished by a high temperature, the coincidence becomes very striking,

The range of the barometer is from 30.82 inches to 23.12 inches; the range of the dew-point from 70° to 11°. The pressure of the vapour varies with it from 0.770 inch, to 0.103 inch. The maximum temperature of the air is 90°, the minimum 11°.— The force of radiation from the sun averages 23.3° in the day, and the force of radiation from the earth at night 4.6°: the highest temperature of the sun's rays, is 154°, and the lowest temperature on the surface of the earth 5°. The greatest degree of dryness is 29°, or the least degree of moisture upon the hygrometric scale 389. The time of the day influences in some degree all the mean results. One of the most constant effects is that produced upon

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the barometer. The mercurial column reaches its maximum height in the morning, declines to its minimum in the afternoon, and again rises at night. The average difference of these periods, as exhibited by the journal, are as follow:—Morning above night, + .005 inch.—Afternoon below morning, — .015 inch.—Night above the afternoon, + .010 inch. —The means of the monthly observations, present but one or two exceptions to the fall in the middle of the day, or to the rise from afternoon to night, but the rise from night to morning is not quite as constant.

With respect to the dew-point, it may be considered that the journal includes four daily observations; for the observation of the minimum temperature of the air, which constantly falls a few degrees below the term of precipitation taken in the day, must obviously be included. From morning to afternoon, it rises but 0.3 of a degree; from afternoon to night, it falls 0.9 of a degree, and below this again, the minimum temperature is 2.7°, and the mean is calculated from the latter, and the afternoon observation.

The temperature of the air varies in the twenty-four hours from 56.1°, its mean maximum, to 42.5° its mean minimum. The mean temperature of a climate, is generally regarded as made up of the average impression of the sun due to its latitude, upon the surface of the globe. The mean quantity of aqueous vapour must also be referable, finally, to the same principle. But there is another way of considering the subject more accurate in detail, though upon an average of years ending in the

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same conclusion: that is, to regard the mean temperature as made up of the temperature of different currents flowing from different points of the compass; and it will be necessary to my purpose to contemplate the atmosphere of vapour particularly, in this point of view. The medium dew-point 44.5° is therefore made up of the following proportions of the means from eight points of the wind—

87 North 40.1°—133 north-east 40.7°.

80 East 42.3°—111 south-east 45.6°.

70 South 48.7°—225 south-west 48.6°.

215 West 44.8°—174 north-west 41.3°.

Before I enter upon the consideration of the effect of the sun's progress in declination, and the succession of the seasons, I shall endeavour to point out the influence of the geographical situation of the island of Great Britain upon its aqueous atmosphere. The mean quantity of the vapour follows exactly the changes of the mean monthly temperature, that is to say, the dew-point rises and falls with the increase and the decrease of the heat. But the winds which transport the vapour may be divided into two classes; namely, the land winds which blow from off the great continent of Europe, and which comprise the north-east, the east and south-past; and the sea-winds which blow from the great oceans which surround it on every other side; namely, the north, north-west, west, south-west, and south. In the former, we may expect to find that the course of the mean temperature is exactly followed; for the sources of the vapour must be comparatively shallow streams, and reservoirs of water, whose temperature must soon adapt itself to that of the sur-

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rounding air. But in the unfathomable depths which supply the latter, the law by which the density of water is regulated, must, at particular seasons, maintain a temperature above the mean of the declining season; whilst at others, the increasing heat of the latter must outstrip the progress of the former. The following Table contains the dew-point of the several winds, divided into the two classes for every month in the year, beginning with the autumnal quarter.

TABLE I. Shewing the Difference of the Dew-point in the Land and Sea-winds.

Land Winds. NE. E. SE. Sea Winds. N. NW. W.SW. S.
September o
53
o
53
October 45 46
November 41 42
December 31 37
January 29 35
February 31 35
March 34 38
April 45 42
May 47 44
June 54 54
July 52 55
August 56 57

And here the effect anticipated is clearly perceptible. The vapour of the land-winds, it will be seen, declines in force from September to January, in which month it reaches its minimum, and from

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that point gradually rises till it reaches its maximum in August; and this, it will be afterwards seen, is the exact progress of the mean temperature of the air. In the sea-winds the vapour follows the same course from September to November, and the balance is such, that the elastic force of both divisions is nearly the same. The north and south winds neutralize each other, and the north-west, west, and south-west, are equivalent to the north-east, east, and south-east. Having descended to about 40°, which is somewhere about the point of the greatest density in water, in November, the accordance proceeds no further. In December, the vapour from the land has descended six degrees below that from the sea, and the difference continues in January. In February the former rises two degrees, and the latter remains stationary. The difference of four degrees continues through March, and is diminished to three degrees in April and May. In June, they again attain their former equality. The reason of this is obvious; the temperature of 40°, being that of the greatest density, cannot be lowered till the whole mass of the waters has passed this term; and in the deep seas, this must necessarily be a process of some duration. The shallow waters, on the contrary, soon assume the temperature of the ambient air, and continue to decline with it in heat. Upon the return of spring the contrary effect is produced. The great deeps must again repass the fortieth degree before the superficial waters can take the higher temperature of the incumbent atmosphere. The consequences we should expect from this progression, would be

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an increase of humidity in December and January, and a rapid decrease in the four following months; an expectation which we shall find correct in our further investigation.

There is another law of the aqueous fluid, which we might also expect to have an influence upon the emission of its steam—the evolution, namely, of heat in the process of congelation and its absorption during the liquefaction of ice. The British isles are placed in such a position, as would induce us to suppose that, at particular seasons of the year, this influence might be perceptible in one direction more than in any other. We may bring this idea to the test, by comparing together the northerly and southerly winds, as is done in the following Table:—

TABLE II. Shewing the Effect of the Ice in the North Seas upon the Dew-point.

Southerly. SW. S. SE. Northerly. NE. N. NW.
September o
58
o
48
October 51 41
November 47 37
December 42 32
January 38 31
February 36 31
March 42 32
April 47 40
May 51 41
June 58 50
July 58 50
August 60 54

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Here we may observe, that the decline of the vapour from September to December is exactly equal in both classes, but from that time it ceases about the temperature of 32° in the northerly winds, and continues in the southerly to the month of February. In March, again, the temperature of the latter has increased from the minimum 6°, but in the former it still remains at 32°. In April, on the contrary, the increase in the northerly winds exceeds that of the southerly; and in May, they have again attained their original relative distances, and resume their parallel progression. It would be difficult, I think, to assign any other cause for this modification of the phenomena than the one which has just been suggested. The evolution of heat, in the process of freezing, stops the decline of temperature in the regions exposed to its influence, while it proceeds in those which are not exposed to the change; and the absorption of heat, in the operation of thawing, prevents the accession of temperature, which is due to the returning influence of the sun. When this operation has ceased, the vapour quickly attains its former relative degree of force.

Wonderful adjustments these, to mitigate the rigours of a northern climate! They both operate from November to February, by the evolution of heat in the coldest season of the year; and at the same time, by an extra supply of vapour, decrease the degree of dryness, and prevent the consumption of heat which always attends the process of evaporation.

Let us now endeavour to trace the order from which, "while the earth remaineth, seed-time and

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harvest, and cold and heat, and summer and winter, and day and night, shall not cease."

In the month of January, the first month of the year, but which, in the most natural division of the seasons, constitutes the second month of the winter quarter, heat is at its minimum in all its particulars. The mean temperature is 36.1°, varying from 39.6°, the mean highest, to 32.6°, the mean lowest; the utmost range of the thermometer being from 52° to 11°. The average power of the sun is 4.4°, and the utmost intensity of its rays 12°. The cold, produced by radiation from the earth, is 3.5°, and the greatest effect 10°.

The mean force of the vaporous atmosphere is also at its lowest point, 0.234 inch, the dew-point being 34.3°. The mean degree of dryness, calculated from the mean temperature and the mean dew-point, is 1.8°, and the state of the air's saturation 939. The average degree of greatest daily dryness is 3.5°, and that of least saturation, 878.

The quantity of rain in this month greatly exceeds the amount of evaporation, the former being 1.483 inch, and the latter, at its minimum, 0.413 inch.

The height of the barometer is 29.921, and its mean range 1.60 inch.

In the month of February the mean temperature increases to 38°, nearly two degrees. This accession takes place principally while the sun is above the horizon; the maximum temperature rising to 42.4°, nearly three degrees, while the minimum only advances about one degree to 33.7°. This difference is partly owing to the increased influence of ra-

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diation under a less clouded sky, which dissipates the accumulated heat; the temperature of the radiant thermometer averaging 29°, one-tenth of a degree lower even than in January. The greatest force of radiation is 10°, as before, but the average effect is increased to 4.7°. The power of the sun rises to 10. 1°, and its greatest intensity to 36°. The range of the diurnal temperature of the air is from 53° to 21°.

The dew-point advances to 34.9°, only 0.6 of a degree; the peculiar laws of the evaporating fluid keeping it back, as before explained. The force of the vapour is 0.239 inch. The consequence of this retardation is, that the mean degree of dryness advances to 3.1°, and the hygrometric state of the air falls to 905. The average degree of greatest dryness is 6.1°, and that of least saturation 816.

The quantity of rain is at its minimum, being 0.746 inch, which is very little more than 0.733 inch, the amount carried off by evaporation.

The mean pressure of the atmosphere is 30.067 inches, and the range of the barometer 1.36 inch.

With the month of March commences the spring quarter, the seed-time of the husbandman, when it is so important to the interests of agriculture that the superfluous moisture should be exhaled from the earth, which would prevent the proper preparation of the soil, and destroy the germinating principle of the grain. By a wise Providence, therefore, the temperature of this month advances six degrees, while the dew-point rises only four, checked by the same cause which began to restrain

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it in the last month. The mean temperature is 43.9°, and the point of precipitation 39°; making the degree of dryness 4.9°, and reducing the moisture of the air to 831. The elasticity of the vapour is .272 inch. The evaporation is rather more than doubled, amounting to 1.488 inch, and exceeding the quantity of rain, which is 1.440 inch. The average degree of greatest dryness is 9.6, and that of least saturation 715.

It is still during the day that the heat accumulates most, the maximum rising to 50.1, and the minimum to 37.7, an increase of 7.9° in the former, and only 4°. in the latter. The temperature of the air ranges from 66° to 24°. The amount of radiation is 5.5°, an increase of nearly one degree, but its maximum effect is 10°, as before. The force of die sun's direct rays is 49°, and their mean maximum effect 16°.

The height of the barometer is 29.843, and its range 1.26 inch.

In April, the mean temperature of the air rises six degrees to 49.9°, and the constituent temperature of the vapour only 4.5° to 43.5°, making the amount of dryness 6.4°. The degree of moisture is consequently no more than 783. The mean of maximum dryness 12.8°, and the mean of minimum saturation 651. The elasticity of the vapour .322 inch. Evaporation is increased to 2.290 inches, and the quantity of rain does not exceed 1.786 inch. The power of radiation from the earth is raised to 14°, and its mean effect attains its highest amount of 6.2°. The power of the

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sun averages 28.1°, and the highest observed effect is 47°. The heat of the air ranges between 74° and 29°.

The mean height of the barometer for this month is 29.881 inches, and its average range 1.11 inch.

In May the temperature of the air still outstrips the advance of the vapour, and the atmosphere attains very nearly its state of greatest dryness. The mean of the former is 54°., that of the latter 46.1°. The state of saturation 769, the degree of dryness 7.9°, the mean minimum of the former 597, the mean maximum of the latter 15.6°. Elastic force of the vapour .354 inch. Evaporation amounts to 3.286 inches, and rain to 1.853 inch. The power of the sun is 57°, its mean greatest influence 30.5. The force of radiation, from the surface of the earth, is 13°, its nightly effect 4.2°. The reduction of this effect implies a rather more clouded state of the atmosphere than that of the preceding month. The mean maximum of the air is 62.9°, the minimum 45.1°: the range of the thermometer from 70° to 33°.

The height of the barometer is 29.898, its range 1.09 inch.

In June, the first month of the summer quarter, the advance of the dew-point and of the daily temperature are nearly equal: the former averages 50.7° the latter 58.7°. The degree of dryness is therefore 8°, and the state of the air's saturation 762.

The force of the vapour .410 inch.

The quantity of evaporation rises a little above

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that of the last month, and amounts to 3.760, the maximum of the year, and the quantity of rain is 1.830 inch.

The energy of the sun's beams is at its height, and also its maximum effect: the former amounts to 65°, and the mean of the latter 39.9°. The temperature of the air does not attain its maximum till the two following months. This arrangement must have an extremely important influence upon the fructification of the vegetable kingdom, and the horticulturist and botanist would do well to attend more particularly, than has hitherto been done, to the different modifications of heat of radiation and heat of temperature. Experience has suggested many practical precautions and artifices evidently connected with this subject, and it is almost certain that a scientific attention to these particulars would tend much to the benefit of the art of gardening.

The force of radiation from the earth, I have once observed in this month to be 17°, the greatest effect that has ever come under my notice: its mean amount 5.2°.

As connected with the subject to which I have alluded above, it is worth while to notice that there are but two months in the year, in which vegetation, in particular situations, is not exposed to a temperature below the freezing point. These two months are July and August, and even in them the radiant thermometer descends to 35° and 34°. Thus, a plant might be so situated, in the month of June, as to undergo all the changes of heat from 154° to 30°.

The mean maximum dryness of the month is 16°:

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the mean minimum saturation, 597. The maximum temperature of the air averages 69.4°, the minimum 48.1°, and the greatest difference between the two happens at this time. This difference is evidently chiefly dependant upon the power of the sun, and the time that it remains above the horizon; therefore, like the direct heat of the solar rays, it follows the progress of the sun's declination. The range of the thermometer is from 90° to 37°.

The mean pressure is 30.020 inches, and the mean variation 0.64 inch.

In July, the increase of vapour is rather greater than that of temperature, and both approach their maximum. The mean heat of the air is 61°, and that of the dew-point 54.5°. The force of the vapour .468 inch. The degree of dryness is 6.5°: the hygrometric degree, 811. Mean maximum dry, ness of the day 13.7. Mean minimum moisture 658. Evaporation decreases to 3.293 inches, and the rain attains its maximum quantity 2.516 inches.

The increase of the mean temperature here appears to be wholly derived from the night, for the mean maximum is only 69.2°, while the mean minimum has risen to 52.2°. This must be owing to the cooling power having been checked by a cloudy sky, and accordingly we find that the effect of radiation has fallen to 3.6°, while its greatest power is 13°.

The force of the sun's rays decrease to 52.5°, but it is probable that this is not their utmost power, as the following month exhibits an increase. Their

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average greatest effect is 25.8°. This decrease of the solar power does not immediately check the mean temperature, for the earth having become heated in the preceding months, acts as a warm body on the atmosphere, and gives out again the heat which it has received. Mr. Howard's explanation of the mean temperature always being about a month behind the sun's place in declination, is, no doubt, as correct as it is ingenious; namely, that "as the sun advances in north declination, the heat we derive from him increases, actually in proportion to his altitude, but not sensibly; because a part of it is required to heat the earth, and is lost there by absorption. As he declines southward in the autumn, the heat we receive actually grows less in proportion, but not sensibly; because we now receive back a certain quantity from the warm earth."

The greatest range of the temperature of the air for this month is from 76° to 42°.

The height of the barometer is 29.874 inches, and the mean range 0.79 inch.

The particulars of the month of August remain much the same as those of the month of July. The warm nights continue, and the heat of the day is undiminished. The mean temperature is 61.6°; the maximum of the day, 70.1°; and the minimum of the night, 52.9°. The range of the thermometer from 82° to 41°. The force of the sun's rays 59.5°, and their average maximum effect 33.1°. The power of radiation from the earth 12°, and its mean amount 5.2°.

The dew-point is 55.3°, and the elastic force of

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the vapour .481. The degree of dryness 6.3°, and state of saturation 819. Mean maximum dryness 12.4°; mean minimum moisture 677.

Evaporation is the same as in the last month, 3.327 inches, but the rain is decreased nearly one-half; the amount being only 1.453 inch.

Mean height of the barometer 29.891 inches; mean range 0.73 inch.

In September, the first month of Autumn, the reduction of temperature begins to be sensibly felt; but, still, less in the night than during the day. The mean temperature declines to 57.8°; the maximum to 65.6°, and the minimum to 50.1°; the greatest range of the thermometer being between 76° and 36°.

The mean dew-point is 52.3°, and the elasticity of the vapour .432 inches; the dryness of the air 5.5°, and its state of saturation 827. Mean maximum dryness 11.1°; and mean minimum moisture 702. The precipitation and evaporation are again nearly upon a par, the former averaging 2.193 indies, the latter 2.620 inches. The power of the sun is but little decreased, its greatest energy being 54°, and its mean daily amount 32.7. Terrestrial radiation also remains nearly the same, rising to 13°, and averaging 5.4°. The height of the barometer is 29.931 inches, and its mean range 0.88 inch.

In October, the mean temperature falls nearly 9°, and does not exceed 48.9°; the maximum and minimum averaging, respectively, 55.7° and 42.1°. The dew-point declines almost in the same proportion

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44.8°. The dryness is reduced to 4.1°, and moisture increases to 870. Evaporation decreases to 1.488 inch, while the rain continues in nearly the same quantity; the amount for the month being 2.073 inches. Now, that the fruits of the earth are laid up in store, this increase of wet is attended by no injurious effects; the remaining heat of the earth is preserved from a needless expenditure, and guarded from dissipation by an increasing canopy of clouds. The effect of radiation is reduced to 4.8°, and its greatest force to 11°. The power of the solar rays declines to 43°, and their mean effect to 27.5°. The greatest range of the air's temperature is from 68° to 27°. The mean elasticity of the vapour is .336 inch, the pressure of the whole atmosphere 29.774 inches, and the average range of the barometer 1.38 inch.

In the dark and dreary month of November, the atmosphere is nearly saturated with moisture. The temperature of the air is 42.9°, and the dew-point averages no lower than 40.5°; the dryness is, therefore, only 2.4°, and the dampness amounts to 910. The precipitations are augmented to 2.400 inches, and only 0.770 inch is carried off by evaporation. The maximum dryness of the days is but 4.7°, and the least degree of moisture 845. The effect of the sun's rays, whose greatest power is 23.5°, is scarcely 6.8°, and that of terrestrial radiation only 3.6°; its intensity being 10°.

The mean highest point of daily temperature is 47.5°, and the mean lowest 38.3°; the utmost range of the thermometer being from 62° to 23°.

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The mean elasticity of the vapour is .286 inch; the pressure of the whole atmosphere, 29.776 inches, and the range of the barometer 0.92 inch.

The month of December closes the year with nearly the same characters as those of the last month; mean temperature, 39.3°; mean maximum, 43.2°; mean minimum, 35.4°; greatest range, from 55° to 17°.

The greatest force of the sun's rays, 12.5°; their mean influence, 5.4°; power of terrestrial radiation, 11° mean effect, 3.5°.

Temperature of the dew-point, 37.6°; degree of dryness, 1.7°; and state of saturation, 952; mean maximum dryness, 3.3°; mean minimum moisture, 888.

Amount of precipitation, 2.426 inches; of evaporation, 0.516 inch.

The elasticity of the vapour, .261 inch; pressure of the atmosphere, 29.693 inches, and range of the barometer, 1.13 inch.

I have not, in the preceding summary, noticed the prevalent winds of the several months, or distinguished the quality of the vapour transported from the different quarters of the compass. I have thought it better to separate this view of the subject from the preceding, and to present the results in a tabular form. The following Table exhibits the average number of days on which the different winds blow in each month of the year, together with the mean dew-point of the vapour which is wafted by them:—

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TABLE III., Shewing the Dew-Point of Eight different Winds in each Month, and the average Number of Days on which each prevails.

N. N.E. E. SE. S. S.W. W. N.W.
No.of Days. Dew Point. No.of Days. Dew Point. No.of Days. Dew Point. No.of Days. Dew Point. No.of Days. Dew Point. No.of Days. Dew Point. No.of Days. Dew Point. No.of Days. Dew Point.
January –31.5 –27.5 –23.5 –34.5 –39. 42.5 –37. –32.
February –30.0 –29. –32. –34.0 –37.5 5 39.5 –39. –34.
March –39.5 4 –31. 2 –35.0 –47.0 –44.5 42. –35.
April –40. –40.5 3 –45. –49.0 –47. 4 –45. –44. –42.
May 3 –42. 4 –40.5 –45.5 4 –54. 1 –54. 6.¼ –49.5 –46 5 3 –41.
June 5 –49.5 –49.5 2 –56. 4 57. 1 –62. –56. 3 –52. 5 –50.5
July –50. 3 –49. 2 –50.5 4 –58. –58.5 7 –59. 5 –56. –53.
August 1 –55.5 –53. –55.5 3 –60 –63.0 6 –58.5 11½ –55. 3 –53.
September 2 –45. 4 –50. 1 –52. 4 –56. 1 –61. 6 58. 6 –54. 6 –49.5
October 3 –38.5 –41.5 2 –45.5 –49. –53.5 –50.5 5 –46.5 –43.
November 3 –38.0 3 –37. 3 –40. 2 –46. 3 –48.0 6 –47.5 5 –42. 5 –35.5
December 1 –31.5 –29. –27.5 4 –38. 2 –45.5 –44 6 –40. 4 –35.
30½ –40. 45் –39.5 26½ –43. 39 –47. 28¾ –51. 73¾ –49. 70¼ –47. 55¾ –42.

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This Table is constructed from the observations of the morning, afternoon, and night, leaving out those of the minimum temperature; and therefore the total means differ slightly from those already given. This has been done for the sake of forming a standard of comparison, whereby to judge of the state of the weather from hygrometric observations. The mean monthly temperature of the dew-point affords a useful criterion for this purpose, but the average state of each wind is much more accurate; and when the Table shall have been improved by the results of a longer series of experiments, an almost infallible judgment may be formed from it of the probability of atmospheric changes. I shall return to this subject hereafter.

It will be observed in the Table, that the northerly winds and the southerly are in nearly equal proportions, but that the westerly are to the easterly nearly as two to one. These proportions are preserved in the several quarters of the year.

It is also worthy of remark, that the dew-point of the sea-winds, viz., the S.W. W. and N.W. is 3° higher than that of the land-winds from the opposite quarters, viz., N.E. E. and S.E.

Thus much we have learnt from the mean observations of the three years together: let us endeavour to advance a few steps further, by a comparison of the years, one with another, and with the mean.

To render this analysis more practically useful, I shall endeavour to connect the popular description of the weather with the scientific details; and I shall, more particularly, aim at elucidating the influence of atmospheric changes upon those branches

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of vegetation, with which are connected the wealth, happiness, and subsistence of the community. By connecting together the reports of the agriculturist, and the observations of the meteorologist, we may, in time, obtain some insight into the nature of the various blights which affect the different products of the soil, whether it be the mildew and smut of wheat, or the, so called, fly of the turnip or the hop. Something may be learnt from a comparison of a forward or a backward season, and the effect of weather upon the soil; some precautions may be suggested, and the grounds of anticipation strengthened. Knowledge, in short, in this, as in all other instances, will surely end in power; at all events, a running commentary, in the common language of the farmer, may draw the attention of those most interested in the subject, to details which might otherwise appear uninteresting or unintelligible.

The observations of the Journal commence with the first month of the autumn of 1819. The particulars do not differ essentially from the mean in any respect. The weather was fine, warm, and seasonable, and corn-harvest was completed under favourable auspices. With respect to the aggregate products of the earth, the season was one of the most plentiful that had occurred for some years. In the early part of the month rain was greatly wanted, and water for the cattle; the pastures were burnt up, and the stubbles bare. The showers in the latter part produced a very beneficial effect.

The results of October, taken together, present nothing very remarkable but the unequal distribution of the fine weather. The first part of the

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month was much above the mean in temperature, and the latter part as much below; and a considerable quantity of snow fell on the 21st of the month, which is very unusually early in the season. The thermometer ranged from 68° to 27°, which are its greatest limits for the month. The degree of dryness was altogether below the mean. Southerly and westerly winds prevailed till the middle of the month, and afterwards the north and north-easterly winds set in; and it was probably owing to their influence that the cold precipitations of the vapour took place. The ground was in a very favourable state for the sowing of wheat, and the turnips derived considerable improvement from the increase of moisture, and were deemed, upon the average, a fair crop. Grass and fodder were superabundant.

The prevalence of catarrhal, rheumatic, and other inflammatory disorders, together with affections in the bowels, were ascribed by physicians to the extraordinary vicissitudes of the weather.

The month of November was colder than had been known for many years. The mean of the thermometer was 3.7° below the season, and it descended to its lowest limit for the month. It was likewise very damp, the degree of moisture being 941, while the average is not above 845. The amount of rain was not great, but that of evaporation very small. The precipitations fell chiefly in light showers, and seldom lasted the whole day.

This season of the year affords but little room for remarks upon the influence of the weather upon the important processes of vegetation. The young

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wheats, which looked generally promising, received a sudden check from the early frosts, and were not expected to benefit from the change to extremely mild and moist weather, which suddenly came on in the latter days of the month. Green food for cattle still abounded.

With the month of December commenced, what may be deemed, a severe winter. The temperature averaged 5° below the mean, but its state of saturation did not differ much from its usual proportion, and the precipitations were altogether light. The mean state of the barometer was rather low, but its range uncommonly small for the time of year.

The long continuance of frost put an end to all the business of the field, but the young crops were well secured by a seasonable covering of snow. The turnip crops, however, received injury in proportion to the benefit derived to the wheat. The alternations between frost and thaw were particularly injurious to that useful root.

In January, 1820, the mean temperature was almost six degrees lower than usual, and the frost continued, with nearly unremitting rigour, till the 23d of the month. The barometer fluctuated very much, and its maximum was higher than had been observed for many years. The weather was altogether damp and unpleasant.

The wheats, and all arable lands, were considered to derive a full portion of the advantages which never fail to accrue from a frost of some duration, accompanied with a sufficient cover of snow. The young clovers, and crops of that description, suffered greatly from the rigour of the

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frost; and vast quantities of turnips were totally destroyed.

The vicissitudes of the weather, and extraordinary severity of the season, produced a more than ordinary number of those disorders which affect the pulmonary organs.

The month of February was still below the mean in temperature, and unusually damp. The amount of precipitations was large, and evaporation very trifling. In other respects, there was no variation worthy of remark.

March was more than usually dry, and there were scarcely any precipitations of rain and snow. The temperature continued lower than usual.

The lands were in a good and fertile state, and the sowing of the Lent corn, was carried on with expedition. The effect of the preceding frosts was to pulverize the soil, which turned up in most places like garden-mould. The latter sown wheats appeared weak, and dependant entirely upon the genial nature of the coming spring. The early wheats stout and healthy. All had, however, suffered in some degree from the sharp N.E. winds, which retarded vegetation, particularly of the grass. The green crops were affected severely.

The mean temperature, in April, nearly recovered its proper amount, and the weather was altogether dry and seasonable.

The sowing of the corn was never completed under more happy auspices. The season had been previously most favourable to manuring, and the heaviest clays worked admirably. Vegetation, which had suffered very much, recovered with the

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timely showers of this month, but many crops, upon loose and spongy soils, were injured past recovery. On the whole, however, the appearance of the country was favourable, and the shew for fruit abundant.

The month of May was genial and seasonable in every respect. The temperature rather above the mean. Rain was interspersed in just such proportion as seems most conducive to the welfare of vegetation. The season, however, was above the mean in dryness. Vegetation was sudden and rapid, and an appearance of luxuriance took place in all the productions of the soil, although the fruit blossoms suffered partially from the frost in the early days. The pulse, artificial grass crops, and hops, which were beginning to suffer severely from their peculiar blight-insects, were well washed by the showers, and speedily recovered the incipient damage.

June was extremely remarkable for being unusually cold and wet in its commencement, and, for the very extraordinary rise of temperature in its latter half. This rise was not accompanied by a proportionate increase in the quantity of vapour, and the weather was beautiful in the extreme. The temperature of the whole period was above the mean, and rain very abundant. The variation of the barometer was great, and its mean very high during the hot weather.

The cold and wet weather increased the bulk of vegetation, but without proportionally accelerating the fructifying process; but after the change, the wheats blossomed most beautifully. Hay-harvest

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was at its height, and an abundant crop most successfully secured.

That precarious plant, the hop, appeared strong and luxuriant.

July was above the average in heat, but a want of sun was experienced to ripen the corn, and a consequent fear of mildew was prevalent. The spring crops and hops were of good promise, and turnips looked very well. The heavy periodical rains of this season commenced on the 16th of the month, and were accompanied at intervals by violent thunder and lightning. Both the quantity of precipitation and evaporation exceeded the mean amount.

August—this month, so important to the agriculturist, was as favourable to the operations of harvest as could be wished: of full average temperature, with a sufficiency of direct radiation from the sun; without which the fruits of the earth scarcely ever arrive at perfection. The mean degree of dryness was greater than ordinary, but a full proportion of rain was not wanting. The weather was particularly propitious till the 19th, when a degree of blighting cold took place, which is marked in the journal by the radiating thermometer falling to 35°. This state of check did not fortunately last long, and was succeeded by a mild temperature, well calculated to forward the labours of the field.

The appearance of the turnips was not so favourable as could have been wished—they had suffered from blight, or from what the farmers term the fly.

With September, 1820, we commence our second annual period, for which we now possess other terms

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of comparison, besides the mean, in the divisions of the year which we have already analyzed.

The temperature of the month was below the mean, and still more below the last year, the difference taking place entirely in the night. This may, probably, have arisen from the great effect of radiation, from a particularly clear state of the atmosphere. The register of the radiating thermometer is in accordance with this supposition; for the mean effect will bé found to be considerably greater than usual, and nearly amounting to 7°. The injury which the hop-plant received about this time, may not improbably be ascribable to this cause: the flower was stinted and small, and mould very prevalent.

This, as well as the last, was one of those plentiful seasons which are not of frequent occurrence. Corn, pulse, and fruit of every kind, abounded. The only draw-back to these advantages was the blight occasioned by the various sudden changes of weather. The usual complaint of fine harvest weather—want of rain for grass and turnips,—was generally heard. The average quantity, however, was not deficient.

October—the temperature of the month was below the mean, and that of last year. Notwithstanding this, the dryness was above the mean, and there was very little rain. The consequence was, great injury to the turnip crops and grasses. The harvest was completed in perfection, by the housing of beans in good condition; and all orchard fruits were very abundant. In many places, the soil was too dry to be worked to advantage, and the sowing of corn was very much postponed till after Christmas.

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November—the temperature was considerably higher than last year, but still a little below the mean; the dryness about the mean. The quantity of rain was very unusually small.

In consequence of this continuance of drought, the turnip crops were generally lost. The young wheats looked healthy and well.

December—the temperature a little above the mean, and six degrees higher than the last year. The quantity of rain was small, but in all other respects, the season did not vary from the average.

Owing to the openness of the weather, a considerable quantity of wheat was put into the ground in the early part of the month. The appearance of the crops was fine, and very luxuriant.

January, 1821, presents a complete contrast to the same month of the last year. The mean temperature was nearly 8° higher, and the degree of dryness much the same. Instead of being bound up in frost and snow, the country, every where, presented an open appearance, and the operations of husbandry were in universal forwardness. Early pulse and beans, had already been planted on the forward lands. Some of the wheat, and other green crops, experienced damage, on clay soils, during the frost for want of a covering of snow.

February—the temperature of the month about the same as last year, and below the mean. The barometer averaged considerably above the mean, and on the 6th, it attained the extraordinary height of 30.82 inches. Scarcely any rain or snow fell, and the season was contrasted with that of last year in being extremely dry.

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The season was most propitious in all respects to the cultivation of the soil. The operations of husbandry proceeded almost uninterruptedly, and the favourable dry season of last year, which occurred in March, was anticipated in this by a month. The wheats, and winter crops in general, had a most promising appearance; so little rain fell during the winter quarter, that moisture was wanting to convert the abundance of straw into manure.

March was a complete contrast to the same month in the last year: very damp and rainy. Though the temperature was very little below the mean, it was cold and unpleasant to the feelings. The seed-time had, however, been anticipated in the preceding month, and the operations completed most favourably. The cold rains of the present month had not any very signal effect in forwarding vegetation, but the crops of every description had a healthful, if not a forward, appearance. The backwardness of the spring is, upon the whole, rather a favourable prognostic for the summer season.

April was of medium character, in most respects, but decidedly warm, its temperature being nearly two degrees above the mean. The warmth and moisture together, began to have their usual influence upon vegetation, which was not, hitherto, in a forward state. The wheat put on a luxuriant appearance; the pulse and Lent corn crops made a very fair shew; and all the operations of husbandry were in a forward state.

The month of May was unseasonably cold and dry, and constituted a very backward season. It was the more irksome to the human constitution as

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well as detrimental to vegetation, from being in occasional contrast, with a day or two of summer temperature. The blossoms of all fruit trees suffered from blight; for the cause of which we need, probably, look no further than the radiant thermometer, which we shall find was on five nights below the freezing point. A sufficiency, however, in most cases, remained for an abundant crop. The shew of grass, lucern, and clover, was great. The wheats appeared strong and healthy, and the spring crops had a thriving appearance. Hops were checked by the cold, but the vine was strong.

June.—The temperature of the month was nearly three and a half degrees below the mean, owing to the prevalence of north-east winds. All hopes vanished of good crops of fruit, and the hops suffered much. The wheats were fortunately backward, and escaped the injury which they would have received had they been in their usual progress, towards the flowering process. The spring corn was retarded in its growth from the same cause, and looked yellow and sickly. The hay-harvest, upon the whole, was good, and turnips, which had been in general early sown, were already beginning to spring up.—The season was rather one of uncomfortable sensation than of positive sickness.

July was deemed an average season, and the different crops upon the ground improved in their appearance. Want of solar heat was very much felt, and high winds had an injurious effect upon the flowering of the wheat; but the crops, generally, appeared in a thriving state. The hops were much mended, and the rains drew up the

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turnips to a size and substance which put them beyond the reach of serious injury.

The month of August, also, did not differ materially from the average. Harvest began about the middle of the month, but in some parts was very backward. Owing to the spring and early part of the summer being unfavourable, and the subsequent beating down of the corn by the rains, the wheats had received considerable damage. Mildew and smut are the diseases consequent upon such seasons as these. Barley, pulse, and tares, were expected to be full crops. Turnips and potatoes very promising.

September was warmer in this year than in the two last, but the degree of dryness very much less, and below the mean. The minimum state of humidity during the day, in the three, was as follows: 675,673, and 759. The quantity of rain was about the usual quantity, but the amount of evaporation one fourth less.

As the two preceding years may be taken as examples of the weather which is most desirable at this season, the present one will furnish a specimen of those expensive and distressing harvests to which the uncertainty of the climate sometimes exposes the farmer. Far worse seasons, however, are by no means of very rare recurrence, in which cold, wet, and blighting weather are more constant. The present was relieved, throughout, by warm and genial alternations. The atmospheric diseases took place early, and the rains, which clouded the harvest, uncompensated by drying winds, completed the misfortune of the crop. The bulk, indeed, was

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great, but samples of good quality very scarce. There was a vast quantity of black and sprouting wheat, and of discoloured barley. On the other hand, the crops of peas and beans abounded, and the country never saw a finer turnip crop, or more luxuriant after-grass. Hop-picking began about the second week of the month, and the crop turned out large, though not of the first quality.

The warm weather still continued in October, but with an average degree of dryness. The crops of turnips covered the land completely, but were considered to be too luxuriant in their foliage. Grass was in great plenty. The worst fears of the farmer, with respect to the grain, were confirmed by its produce upon the barn-floor.

November differed from the mean, and from both the preceding years, in a very extraordinary way. The average temperature was 5° above the usual amount; and, although its dryness was in excess, the quantity of rain exceeded the mean quantity by one-half. The barometer, upon the whole, was not below the mean. All the low lands were flooded, and the sowing of wheat very much interrupted by the wet. The early sown, upon good lands, was very forward and luxuriant. A complaint was made, that the turnips had more foliage than bulb, and the potatoe crops were small, and of a blighted and inferior quality.

In December, the quantity of rain was very nearly double its usual amount. The barometer averaged considerably below the mean, and descended lower than had been known for thirty-five years. Its range was from 30.27 inches to 28,12

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inches. The temperature was still high for the season, and the weather continued, as in the last month, in an uninterrupted course of wind and rain; the former often approaching to a hurricane, and the latter inundating all the low grounds.

The water-sodden state of the soil, in many parts, prevented wheat-sowing, or fallowing the land at the regular season. The mild temperature pushed forward all the early-sown wheats to a height and luxuriance scarcely ever before witnessed. The grass, and every green production, increased in an equal proportion.

January, 1822.—This most extraordinary season still continued above the mean temperature, but the rain, as if exhausted in the preceding month, fell much below the usual quantity in this. There was not one day on which the frost lasted through the twenty-four hours.

The wheats, where they had not been flooded, were generally found to look well; but drawn up as they had been, by warm and moist weather, without the slightest check from frost, serious apprehensions were entertained lest they should be exhausted by excessive vegetation, and, ultimately, be more productive in straw than corn.

The month of February, still five degrees above the mean, ended a winter which has never been paralleled. The dryness, also, above the mean; evaporation great, and very little rain. There was no snow in Middlesex. Under these circumstances the earth became in the finest state for cultivation. The superfluous moisture was exhaled with far less damage from the floods than had been antici-

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pated. The spring culture for every article was forward. The wheat looked finely, and grass was in profusion.

March was again 4° above the mean in temperature; dry, with a proper average proportion of rain. Wheat, taken generally, never looked better; the risk from winter frosts at an end, and nothing to be apprehended but those cold easterly winds, which are generally expected to succeed a mild winter. The appearance of the country, with respect to the lands and the crops, was universally favourable. Great evaporation took place, and the soil was every where getting into the finest state. The grass had a beautiful verdure, and all the spring crops were got in without hinderance from the weather. The shew for fruit, luxuriant and beautiful.

April did not differ much from the general average, but its temperature was low, with a more than mean quantity of rain. The frosts, in the early part of the month, were very injurious to the fruit-blossoms. The wheats suffered in colour and condition from the alternations of heat and cold. All the spring crops had been well got in, though on clay soils, with considerable trouble and expense.

A genial May, of the usual character of that month, visibly improved the crops upon cold, unsound land, which had, in course, suffered most. The appearance, throughout the country, was generally promising. The fallows and lands, for the spring crops, had been worked with much difficulty, owing to the want of the disintegrating influence of frost. All the green crops and grass were of good

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promise. Great complaints were made of the weak and blighted state of the hops, and fruits had suffered materially.

The month of June was unusually dry and hot; evaporation was enormous, and rain very short of the usual quantity. The amount of the former was 4.65 inches, and that of the latter only 1.11 inch.

The aspect of the country was considerably reduced in verdure and luxuriance, by almost constant drought and excessive solar heat. The amount of radiation from the earth was also great, and the radiating thermometer four times descended to the freezing point. There were several thunder showers, but the rain was not sufficient to moisten beyond the surface of the earth, or effectually nourish the vegetable roots. The autumnal wheats flowered in the most beautiful manner, and there was every prospect of an early harvest. The Lent corn crops suffered for lack of moisture. Hay-harvest was completed in the best possible mainner, and the crops were good. The hops improved, and the apples promised well.

The timely showers of July revived the appearance of the Lent corn and pulse, which were generally injured by drought. The oldest person did not recollect either an earlier hay or corn harvest; or more successful ones, to this time, both with respect to weather, and quantity, and quality of produce. The turnip sowing, which had been attempted in the dry weather, was obliged to be repeated after the rains. The crop of potatoes middling.

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The weather continued through August the most beautiful, and best adapted to getting in the harvest, and, indeed, to every agricultural purpose, that could have been chosen.

The wheat crop was safely housed early in the month, and the next article in rank for human subsistence, potatoes, were of equal promise with the wheat, both in regard to quantity and quality. Barley, oats, and beans, were only good in some favoured situations; and, in general, those crops were considerably below an average, though much improved by the showers which succeeded the long drought. Oats, particularly, suffered from smut in many parts. No crop received greater benefit from the rains and subsequent warm weather than the hops. Turnips were a failing crop, destroyed almost entirely by the fly.

The method adopted by Mr. Howard of expressing a series of results, by a curve, is so extremely useful and striking, and so greatly facilitates the comprehension of their different connexions, that I shall here proceed to connect my own observations with his, by a similar occular comparison of certain particulars, and endeavour to supply the void in his graphic illustrations, by tracing upon the same plan, some of the variations of the aqueous atmosphere.

Fig. 1, Plate I. presents a comparison of the monthly mean temperature of the air, and of the dew-point, throughout the year, founded upon the observations of the three years. The full line exhibits the progress of the former, the dotted line that of the latter; and the degree of dryness of the different periods,

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is accurately represented by the space included between the two. From this we clearly perceive how closely the constituent temperature of the vapour follows the mean temperature of the air; but we may, at the same time, remark that it is more speedily diminished by the fall of the latter, than it is increased by its rise. Hence arises the contrast between the dryness of the spring and summer months, and the dampness of the autumn and winter. The two horizontal lines are drawn upon the annual means, and form the standards of comparison for the several variations.

Fig. 2 presents the separate means of the three years, by the conjunction of which the last curves were constructed. The relation of the different lines will be sufficiently intelligible from inspection, without minute description. The character of the various seasons which we have just been analysing and comparing, will be found to be very prominently marked upon this diagram. The wet autumn of 1821, which proved so injurious to the harvest, is characterized by the near approximation of the two dotted lines in September; and the very cold winter of 1820, by the low descent of the continuous lines in the scale in January. The hot and seasonable summers of 1820 and 1822, and the blighting June of 1821, together with the very extraordinary winter of 1821-1822, are also well defined. The oscillations of the seasons, round the horizontal lines of the general means, exhibit many other points of interesting comparison, which will be sufficiently obvious to those who are inclined to take this method of studying the results of the observations. The

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general correspondence in the progress of the mean temperature, and that of the dew-point is sufficiently evident in the separate years; but there are some evident modifications of the general law, presented by these curves, which are well worthy of attentive consideration.

To render tha general accordance between the mean temperature and the dew-point, still more evident, Fig. 3 has been constructed, in which the variations of the daily mean, are represented for forty-five days, in September and October, 1819: and to carry the analysis of this connexion to its greatest extent, Fig. 4 presents the observations four times in the day, for eleven days, including the daily maxima land minima. By inspection of the latter figure, it will be evident that the maxima of the temperature of the air have but little influence upon the force of the vapour, which appears to be governed chiefly by the daily minima; and a line connecting together the points of greatest depression would not differ very essentially from the curve of the dew-point; while a similar connexion of the points of greatest elevation would scarcely resemble it in any particular.

Plate II represents the comparative fluctuations in the elasticity of the air and vapour, and in their temperatures, taken at morning and evening, for a period of three months, in 1819. The former are laid down upon the same scale; and the two upper lines depict the real proportionate differences of the simple and compound pressures. An attentive consideration of these will shew that the undulations are

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generally in contrary directions, and a rise in the line of vapour is generally accompanied by a fall in the barometric curve. The lower lines depict the variations of the temperature: the continuous one that of the atmosphere; the dotted that of the point of condensation. In fine weather, it will be observed that these two are widely separated, while, during the time of aqueous precipitation, they coincide. In this diagram may be distinctly traced the much greater variations of the aqueous atmosphere at the upper part of the thermometric scale, than for equal differences at the lower; and, in short, all the connexions of temperature, moisture, and pressure, may be comprehended at one glance. The converging lines are intended to indicate the direction of the winds.

The preceding pages I would wish to be considered as an Appendix to Mr. Howard's valuable and laborious work; and, as such, I trust that they will not be found to be without interest. There are many points upon which I have refrained from touching at all, which have been discussed, with all the precision which the present state of our knowledge will permit, by that able and indefatigable observer; and for information upon these, I prefer a reference to the work itself, to an attempt to recapitulate his clear and comprehensive statements. The climate of London has altogether received a degree of elucidation, (to which I shall be proud to be considered to have contributed, in however small a degree,) which distinguishes it from any other on the globe, and which, it may be hoped, will excite some

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degree of emulation in other countries. But much remains to be done even in this field of meteorology, and it is probable that the most interesting and practically-useful points, are those which are still enveloped in the greatest obscurity. If the interest which has lately been excited on the subject, and the consequent spirit of observation be properly directed, there can be little doubt that the progress to perfection will be sure and rapid.

X

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METEOROLOGICAL OBSERVATIONS

MADE AT

MADEIRA, SIERRA LEONE, JAMAICA, AND OTHER STATIONS, BETWEEN THE TROPICS,

By Captain EDWARD SABINE, R.A., F.R.S.

MY friend, Captain Sabine, before his departure upon his voyage to the south, in 1822, for the purpose of ascertaining the length of the second's pendulum, at different stations near the equator, with that unaffected zeal for science, for which he is so much distinguished, kindly undertook to try any experiments, or make any observations which I would point out as likely to be of interest to meteorology. I eagerly embraced so desirable an offer; and, not to be backward in promoting to the utmost of my ability such an important object, I sketched out some imperfect directions for his guidance. The results which he communicated to me upon his return, are of the highest interest and importance, and of particular value, from his well-known accuracy. I have already availed myself largely of their general authority in the preceding Essays, and shall now, with his liberal permission, proceed to detail, in his own words, the particulars of his notes. I shall here omit such experiments as relate particularly to the subjects of radiation, and the horary oscillations of the barometer, as they are to be

X 2

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found in the preceding pages. The observations commenced at the island of Madeira.

"The mountainous parts of the interior of Madeira have been rendered accessible to a greater distance than formerly, by roads of recent construction, passable at most seasons by mules, or by the small horses of the island, which vie with mules in the sureness of their footing. I availed myself of the opportunity which our short stay afforded, of making an excursion to the summit of the Pico Ruivo, the highest of the island, with a view to obtain a measurement of its height, and to make a first essay with a portable barometer having an iron cistern, on which Mr. Newman had bestowed much pains, to obviate the liability to the various errors to which these instruments are generally subject. The party consisted of Captain Clavering, of his Majesty's ship Pheasant, Mr. Whitelaw, surgeon of the Iphigenia, Mr. George Don, naturalist of the Horticultural Society, and two midshipmen of the frigate; we were accompanied by Mr. Blackburne, an English merchant resident at Madeira, who, having before ascended the Peak, was kind enough to undertake to conduct us, and by his local knowledge and authority over our Portuguese attendants and guides, as well as by his own enterprising spirit, enabled us finally to accomplish our purpose. Lieutenant Stokes, of the Iphigenia, was so kind as to remain on board the frigate throughout the day, to note the variations in temperature and density of the atmosphere, and the point of deposition indicated by the hygrometer. These were observed hourly by a chronometer, so as to be simultaneous with

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others which we should make at the heights at which we might find ourselves.

"We quitted Funchal before day-break, and proceeded about six miles along the coast to the westward to Camera de Loubos, from whence we commenced the ascent in a northerly direction. At eight we stopped to breakfast at the Jardim de Serra, a house which Mr. Veitch, the British consul-general, has built, at an elevation of nearly 2,800 feet In approaching this height, the vegetation reminded us at every step of England; the people of the country, whom we met on their way to mass, impressed us favourably by their courteous demeanour towards each other, as well as to strangers; they were well, and even handsomely, clothed; the men able-bodied and good-looking, but the women, almost without exception, very plain.

We found the temperature at Mr. Veitch's 16° less than at Funchal, being a much greater difference than we had expected as due to the elevation. An ascent of about half an hour from the Jardim opens the first sight of the Curral, which struck me, who am, however, but little accustomed to mountain scenery, as the most magnificent view I had ever seen; the Curral das Freiras, which means literally, I believe, the Sheepfold of the Nuns, is a ravine extending several miles in a north and south direction, and of considerable width, the sides extending four thousand feet in height, in character frequently precipitous, and where so, being in fine contrast with the deep green foliage of the trees, by which the sides are more generally clothed; these trees are principally laurels, amongst which we noticed

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the Nobilis, Indica, and Fœtens. The valley of the Curral is occupied by a small river, which descends from the high land of the interior with all the character of a mountain torrent. Our route led into the Curral, for the purpose of ascending its valley, but the descent being impracticable at the spot where the first view is obtained, the road continues to ascend, passing over an elevated ridge, on which there was much snow. In descending, on the Curral side of this ridge, and at some distance beneath its summit, is a copioas spring, which collects in a shaded basin formed in the rock by the workmen by whom the road was made. The temperature of the water in this basin was 47.2°, that of the air 46°, and at Funchal 65°; its elevation 4454 feet.

Whilst these observations were making, the summit of the Pico Ruivo, which was enveloped in clouds during the day, was visible for some minutes; and it may be worthy of notice, that this was the only period in which the proportion of moisture, in the upper air, to saturation, was observed to be less than at Funchal. The wind throughout the day was easterly and light, but with little of the unpleasant sensation which usually characterizes the Leste.

The time pressing, we committed our horses to the Portuguese attendants, and descending ourselves on foot, more quickly than we should have done on horseback, although stopping occasionally in admiration of the splendid scenery on every side, which it was impossible to pass without notice, we crossed at noon the Ribeiro di Curral, on a tree which had fallen across the torrent, the horses fording it lower down, and pursued a road which led to the head of

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the valley. We there recommenced the ascent, and passing through districts of brooms and ferns, entered the snow at a somewhat lower elevation than on the heights near the coast. At two P.M. we reached the highest point attainable on horseback, by reason of the depth of snow, and of the frequent quebradas, or breaches, in the road, caused by the descent of torrents. It is a ridge 4380 feet above the sea, over which the road passes at the foot of the Pico das Torrinhas, which is inferior in height only to the Pico Ruivo. From hence, Mr. Whitelaw and myself proceeded on foot, the others of our party returning to the valley to await us. Entering a thick wood of evergreens, consisting of laurels, of the quercus ilex, and of the erica arborea, which attains a large size, and grows even at the summit of the mountains, we were soon enveloped in the clouds by which the Peak was hid from our sight; and, after an hour and a half's good walk through snow, which latterly exceeded two feet in depth, impeded occasionally by the quebradas which are passable only by the aid of roots and branches of trees, and not without danger, as a slip unrecovered would generally be fatal, we attained the summit. We experienced no other inconvenience than being wet by the rain, and a little cold, whilst we remained to make the necessary observations to ascertain the height; certainly none that need deter others from a similar undertaking at the same season of the year, when, should the weather be clear, they will be amply repaid. The Peak being nearly in the centre of the island, the view from it must be very splendid, though of this we were only

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able to form an imperfect judgment from the unfavourable circumstances of the weather. It is not otherwise interesting than as relates to its height and situation, being merely one of several pinnacles in an island of volcanic formation.

It was dark before we had rejoined our party in the valley. We had then to reascend the opposite side of the Curral to that which we bad descended in the morning, in order to gain a nearer road to Funchal than by the Jardim de Serra. This asoenl was more precipitous than any we had yet traversed, and made those amongst us feel nervous who had not learned from habit to confide in the sure footing of the horses, inasmuch as, during the greater part of the way, a single false step would have precipitated the horse and rider many hundred feet into the valley beneath; the apprehensions of danger were, perhaps, augmented by the accompaniment of torch-light, and induced some of the party to trust to themselves rather than to the horses; we all, however, reached Funchal in safety by midnight.

The barometer was found to answer extremely well, both in conveyance and in use. I am not aware of any objection to the iron cistern to counterbalance its many advantages over those of leather or of wood, the former of which are especially faulty in being affected by damp, whilst the certain freedom of the mercury from air and moisture in barometers of this construction, give them a decided preference over those which are filled on the spot, and which I cannot consider as otherwise than very uncertain. I regret extremely that I had not an opportunity of

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ascertaining its performance in the more important ascent of the Peak of Teneriffe, but our departure from England had been so long delayed by contrary and tempestuous winds, that we were only able to remain seven hours at Santa Cruz. We were told, indeed, that the Peak was inaccessible in the winter season, but we had heard the same at Fundial of the Pico Ruivo. I am aware that the difficulty in the two cases does not admit of comparison, but the true interpretation is, that neither is accessible without more exertion than travellers are ordinarily disposed to bestow. Had Sir Robert Mends felt at liberty to have remained at Teneriffe for three days, we should certainly have made the attempt; and as Captain Baudin succeeded in December, I trust we should not have failed in January. The precise determination of the height of this peak is yet to be accomplished, and appears worthy of being undertaken, were it only to submit barometric measurement to the test of a more exact comparison with the geometric method, (both conducted with the precision of which modern instruments are capable,) than has yet been effected. A residence of some days, at the proper season, near the summit of this remarkable Peak, winch rises so abruptly, and to so great an elevation, from the middle of the basin of the Atlantic, might indeed be expected to produce many important meteorological and other results; and would certainly throw much light on the extent of variation, to which barometric measurement is liable, from varying circumstances connected with the atmosphere itself, independently of errors of instrument or observation, or of the formula by which

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a result is deduced; the limit within which this liability might be apprehended, would appear, by a comparison of the registry of the barometer at the top and at the bottom, continued for a sufficient time.

We experienced a similar disappointment, and scarcely in an inferior degree, in passing hastily by Fuego, one of the Cape de Verds. I am not aware of any good account of this very remarkable island having been published, and am surprised that it has been so little visited. It rises in a cone almost from the water's edge, to an height much exceeding that of St. Antonio, which is estimated by Captain Horsburg at 7400 feet, and we had reason to conclude, from the angle which it subtended at different distances, justly estimated. The summit of Fuego was visible from the ship for two days, rising much above the clouds, and always clear; no smoke proceeded from it, although it is said to be generally burning. I cannot conceive a station more eligible for interesting experiments, connected with the relations of heat and moisture to the atmosphere.

The detail of the observations, and the heights computed from them, is as follows:—

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Observations made at Madeira, January 13, to determine the Elevation of several Stations in the ascent to the Pico Ruivo.

STATIONS. Observations at the Station. Corresponding Observations 8 feet above the Sea.
Barometer. Temperature. Point of Deposition. Barometer. Temperature. Point of Deposition. Height deduced.
Air. Mere. Air. Mere.
Jardin di Serra, floor of the upper story of Mr. Veitch's house Inches.
27.681
o
49
o
49
o
41.5
Inches.
30.603
o
65
o
65
o
54
Fest.
2782.6
Basin of the Spring 26.012 46 46 34 30.543 65.5 65.5 53 4453.9
Ridge at the foot of the Pico das Torrinhas 25.948 42 42 36 30.423 64 64 56 4379.7
Summit of the Pico Ruivo. The observations were made eleven feet below the summit, but the computed height is that of the summit itself 24.938 36 36.5 36 30.428 61.5 61.5 58 5438.1

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"The results have been deduced in the manner explained in Mr. Daniell's paper, 'On the Corrections to be applied in Barometric Mensuration,' published in No. XXV. of the Quarterly Journal of the Royal Institution; the barometric differences have been augmented by 1/68, as 68 indies of mercury in the tube are equivalent to one inch in the cistern; and 1/500 of the approximate result has been added, as a correction due to the variation in density of the atmosphere, in the latitude of Madeira.

Between Madeira and Sierra Leone.—We entered the trades on the 20th of January, in lat. 24⅓° N., and long. 19°W.; between which, and the Cape Verd Islands, the point of deposition varied from 60.5° at eight A.M.; air 69°, and surface water 68.25°, to 62°; air 70.6° to 71.2° at noon, and six P.M., on each of the 21st, 22nd, 23rd, and 24th, days of January. When amongst the islands, between San Vincente and St. Jago, the breeze being rather fresher than customary, and the weather as usual, clear without haze, the point of deposition was as low as 54.8°, air 71.2°, surface water 71.2°.

In the due-east passage from the Cape Verd Islands to Cape Verd on the Continent, (400 miles,) the point of deposition (as usual at eight, or rather half-past seven, noon and sun-set,) varied from 63° to 64°; air from 70.2° to 71.2°; until at eight A.M. on the 31st, approaching Cape Verd, then twenty-six miles distant, and not in sight, it fell to 61.5°; air 70°; surface water having also fallen from 71° to 69.6°. At noon we were between three and four miles from Goree; deposition 57.5°, air 70°,

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water 64°. At sun-set, again proceeding, more distant from the coast, to St. Mary's, 60.5°, air 69°, water 65°; and on the following morning, at sunrise, anchored in the Gambia River, twenty miles from St. Mary's; point of deposition 50°, (and at noon 51.5°), air 69°, water of the surface 67.5°; and at thirty-four feet depth, 66.5°, being two feet above a hard, sandy bottom. On the 2nd of February we anchored off Bathurst, higher up in the Gambia River; point of depression 48.5° to 49.5°, air 69°, on the 2nd, 3rd, and 4th; but on the 5th, in the forenoon, something approaching to a Harmattan took place, the wind fresh from the N.N.E. to N.E.; the weather hazy towards the horizon; the point of deposition 37.5°, air 66.5°, and water the same. We sailed in the afternoon, when the wind shifted to the N.W., and the point of deposition rose to 60.5°, air 70°, at six P.M.

"Between the Gambia and the River Sierra Leone, we were usually off, and in sight of, the coast; occasionally waiting off the mouths of the larger rivers. Whilst at anchor off Cape Roxo, (four miles distant,) we had the air 68.5° to 67°; point of deposition varying from 56.5° to 60° in the course of the day, (7th and 8th of February;) surface water 68°. On the 9th of February we passed outside the Bissao shoals to the mouth of the River Grande; air 71° to 70.2°, hygrometer 64°, surface water 66.5; and when anchored off the mouth of the Rio Grande, the air was warmer, (on the 10th and 11th of February,) 73° to 74.5°; being warmed by the river water, the surface of which was 75° off the mouth; and more moist, the point of

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deposition being 64° at eight A.M., 66° at noon, 68° at six P.M., 69° at midnight.

Sierra Leone.—From the latter end of February to the beginning of April, at half an hour after sunrise, the point of deposition was usually found from 68° to 71.5°, air 75° to 78°; at which time, and until the sea-breeze sets in, a calm prevails, and causes it to be the most oppressive period in the twenty-four hours. About ten or eleven the sea-breeze commences usually in the N.W., freshening, and becoming more westerly as the day advances; With this wind the point of deposition generally advances about 2° by noon, and the air to the average of 81°: in the afternoon, little or no regular change is perceptible in the point of deposition— not equal to 1° whilst the air advances to 83° and 84°. Towards the evening the sea-breeze dies away, and the land wind gradually springs up; and at ten P.M. I generally found the point of deposition 5° or 6° less than at half an hour, or an hour, after sun-rise the following morning. The average maximum of the hygrometer, daily, was 72.7°; the particular observations varying from 70° to 75°; the minimum may be inferred to have been below 73.2°, which is the average minimum of a register thermometer, suspended a foot above the parapet of the bastion of Fort Thornton, in a fair exposure; and which, consequently, was not so low, as if it had been placed on vegetation on the ground. This average is a mean of twenty nights; the particular observations varying from 72.5° to 74°. On two of these nights the thermometer was placed flat on the ground of the parapet; on two others, suspended

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by a cord, extended from parapet to parapet, across the bastion; in two others, placed in a copper-inverted cone, highly polished on the outside, and blackened within; and in all the other nights, strung across, between the pillars of my transit, (the instrument itself being removed,) on the parapet. The register thermometer was of Six's construction; the cylindrical bulb blackened, and wrapped in black wool; the wooden back hollowed out in the centre. In the various situations it appeared to be alike affected. It being the dry season, there was not a tuft of vegetation (excepting trees) within any moderate distance of the fort. That a greater difference does not take place between a thermometer thus exposed, and one under shelter, measuring, as nearly as may be, the true temperature of the atmosphere, may, perhaps, be caused by the peculiar situation of Free-town, immediately inland of which a ridge of mountains rises somewhat abruptly; and the cold air from hence is brought down immediately to Free-town by the land wind, which prevails at night. Free-town has also the sea on the west, and a river of several miles' width on the north; these circumstances combined, seem to explain the reason, why the usual effects of radiation should be lessened, whilst the air introduced is colder than if the whole country were on the same level.

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Register of the Height of the Thermometer at Sierra Leone, 1822
Hours. Feb 26 27 28 Mar.1 2 3 5 6 7 8 9 10 11 12 22 Mean temperature
of each four hours.
10 A.M. to 2 P.M. Max.
Min.
85
78
86.5
77.5
81
77
81
76
81.1
78.3
85
79
82
79
83
80
82
79
83
78
81.5
80
82.8
78.5
82
79
83
80.5
84.2
84.2
82.85
78.69
80.77
2 P.M to 6 P.M. Max.
Min.
85
79
87
87
80
77
81.5
78
85
78
85
79
83
80
83
78
82
77.73
85.5
81.5
81
78
84.5
78
82.5
78
84
79
86.5
80
83.97
78.58
81.27
6 P.M. to 2 A.M. Max.
Min.
78.5
74.5
79
77
79
74
80
76
80
76.4
79
78
80
79
79.5
78.5
81.5
77
81
80.5
78.8
76.5
79
76
78.5
76.5
80.5
78.5
80
78
79.02
77.09
78.36
10 P.M. to 2 A.M Max.
Min
78.25
74
77.5
73.75
77
72.75
80
77
80
75.5
78.75
77.5
80.25
78.25
79.5
76.75
80
75.75
81
78
77.6
75.5
78
76
78.8
76
81
77
81
77
79.24
76.18
77.71
2 A.M. to 6 A.M. Max.
Min.
78
73.5
76
71.5
75
71.5
80.5
76
80
71.5
79.5
76
80.2
77.25
79.5
75
78.5
74.5
81
75.5
81
75.5
77.5
74.5
77
75.5
78.5
75.5
82
76
78.95
75.02
76.98
6 A.M. to 10 A.M. Max.
Min.
78.5
75.5
78.5
76.25
78.5
72.5
79
75.5
80
77.5
80
77
78.2
75.5
81
77
79.5
75
79
77
81
76
79
76
79.8
77
81.5
78.5
82.4
76.5
79.73
76.2
77.97
True mean daily temp 78.23 78.54 76.3 78.38 78.92 79.48 79.39 79.23 78.54 80 78.79 78.36 78.38 79.79 80.37 78.85
Mean of the extremes 79.75 80.7 76.25 78.5 79.75 80.5 79.25 79 78.25 80.5 79.75 79.5 79 79.75 81.25 79.45

A (Six's) Registering Thermometer was suspended in a copper cylinder, highly polished on the outside bottom, to admit a free current of air; the cylinder itself was suspended fire feet from the g n the parapet of the bastion of the fort: a mean of the register of the thermometer at 8 A.M. it t 9 A.M. as much above it.

[page] 321

Details of a Barometrical Measurement of the Sugar-loaf Mountain.—The Sugar-loaf, so called from its shape, is the highest point of the mountain district of the colony, included as yet within the limit to which cultivation has extended. This district is the site of the twelve most interesting settlements of liberated Africans, from the principal of which, Regent-town, it is distant about three miles, being altogether about eight or nine from Free-town, the seat of government: a road has been opened by the inhabitants of Regent-town, by which the summit is accessible, and has been sufficiently cleared of its forest-trees to admit the view around. In the continuation of the Sierra towards the south, at about twenty miles' distance, the land appears to attain a greater general elevation than in the neighbourhood of the Sugar-loaf, and there are several points, especially, which are probably much higher: to these there is as yet no road, but from the very rapid advance which the colony is making in population and in settlement, it cannot be doubted that these points must very shortly be necessarily included in the Colonial Survey.

Dr. Nicol, deputy-inspector of army hospitals, was kind enough to allow me the use of a stationary barometer, in excellent order, made by Cary, and the property, I believe, of the College of Physicians: it is the same instrument which has since accompanied Captain Laing in his very interesting excursion to the Soolima country, in which the Niger takes its rise, and which has enabled him to ascertain satisfactorily the elevation at which that

Y

[page] 322

river originates its yet unknown course. The accordance of the portable barometer with the stationary was examined before and after the observations for the measurement; the latter was placed in the room in Fort Thornton, in which my pendulum experiments were made, and its height, consequently, above half tide, carefully ascertained by levelling; was known, with tolerable precision, to be 190 feet. The variations in the density and temperature of the atmosphere, and in the point of deposition of moisture, as indicated by the hygrometer, were observed at this spot by Captain Laing, at stated periods, with a chronometer, on the 28th of March, so as to be simultaneous with such as should be made at elevations.

I shall confine myself to stating the data necessary for the calculation of the heights of the clergyman's house at Regent-town, and of the summit of the Sugar-loaf. At the first of these stations, the barometer, having been suspended above an hour, five feet below the gallery which surrounds the clergyman's house, shewed at seven, A.M., on the twenty-eighth March, 29.017 in., th. 74.5°, and the point of deposition 57°; the corresponding observations at Fort Thornton were 29.820 in., th. 79.5°, and the point of deposition 66°. At eleven, A.M., on the same day, the barometer being suspended in the shade, at the summit of the Sugar-loaf, the cistern 1½ foot below the highest point, was suffered to remain until twelve o'clock, that the mercury might acquire the temperature shewn by the attached thermometer; when the observations regis-

[page] 323

tered were 27.560 in., th. 82.2°, and the dew point 70°,—those at Fort Thornton being 29.795 in. th. 84°, and the dew point 70°, also.

"The. mercury being reduced to the same temperature at the upper and lower stations, and 1/68 of the differences in the heights of the column being added, on account of the respective diameters of the tube and cistern of the barometer; the true differences are, between Fort Thornton and Regent-town .8 in., and between Fort Thornton and the Sugar-loaf 2.263 in., at the temperatures of the air, and under the pressure of the amount of atmospheric vapour specified above. The approximate heights due to these differences being corrected for the latter circumstances, it results that the floor of the gallery of the clergyman's house at Regent-town is 983.6 feet, and the summit of the Sugar-loaf, 2521.6 feet above the sea.

"I have taken the liberty to add (though without permission) an extract of a letter which I have received, since my return to England, from Thomas Stuart Buckle, esq., engineer and surveyor of the colony, stating the result of a comparative geometrical measurement. "I was much gratified to find, on computing the altitude of the Sugar-loaf, from the trigonometrical observations that I had taken, that the result differs from your barometrical measurement only a few feet: I make its height 2493 feet; the height of Leicester Mountain I computed to be 1954, and it was sufficiently satisfactory, on taking into account the distance of the Sugar-loaf from Leicester Mountain, and the excess of its height above that of Leicester Mountain, that the result of the

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[page] 324

latter was 537 feet, which, added to 1954, amounts to 2491, differing from the former calculation only two feet."

I shall here add the barometric measurements of well-known places in the islands of Ascension, Trinidad, and Jamaica; but I am not aware of any previous results with which to compare them.

Height of the Mountain-house at Ascension.—July 9th, 1822, at 9h 30m A.M., a barometer, seventeen feet above the sea, in a room in the Barrack-square at Ascension, stood at 30.165 in., the temperature of the air and mercury being 83°, and of the point of deposition 68°; whilst, at the same time, another barometer, three feet above the floor of the Mountain-house, stood at 27.950 in., the air and mercury 70.3, and the point of deposition 66.5. From these data, the floor of the Mountain-house would appear 2221.8 feet above the sea.

The upper barometer was then taken to the summit of the island, but the registry at that height has been mislaid; it was 27.3 and some hundreds, being less than 700 feet above the Mountain-house; consequently, the highest part of Ascension is under 3000 feet: on returning from the summit, the barometer was replaced three feet above the floor of the house, and allowed to remain until the mercury should have acquired the temperature of the air, when, at lh 30mPM., its height was27.937in., air and mercury 72°, point of deposition 68°, and in the lower barometer 30.137 in., air and mercury 84.5, point of deposition 71°, whence the height of the floor of the Mountain-house results 2219 feet above the sea, being three feet less than the first measurement.

[page] 325

The mean, consequently, or 2220.5 feet, is considered the correct elevation.

Height of the Block-houseat Fort George, Trinidad. —October 9th, 1822, at 8h 30m A.M., a barometer, four and a half feet above the foundation of the Block-house, stood at 29.000 in., the air and mercury being 76.5, and the point of deposition 76.5 also, with slight rain. The corresponding height of the barometer, at the same time, in the Protestant church in Port Spain, twenty feet above the sea, was 30.058 in., air and mercury 82°, and the point of deposition 77°. Whence the foundation of the Block-house would appear 1067 feet above the sea.

Height of Mr. Robert Chisholm's House, in the Port-Royal Mountains, Jamaica.—October 31st, at 4h 30m P.M., a barometer, suspended against the wall of Mr. Chisholm's house, two feet above the ground, stood at 25.967 in., the air and mercury being 68.5, and the point of deposition 68.5 also; and on the 2d of November, at six A. M., at 25.963 in., the air and mercury 65°, and the point of deposition 60°. The corresponding observations at Port Royal, at the same hours, eight feet above the sea, were—

Oct. 31, —Bar. 30.007; Air, 82.5; Merc., 84.5; Dew point, 77
Nov. 2, 30,023 78. 78. 72

Whence the height of the ground on which Mr. Chisholm's house stands, results respectively, 4087.9 feet, and 4072.7 feet, the mean being 4080.3 feet above the sea.

"All the observations at heights were made with

[page] 326

the same portable barometer; 1/68 therefore, is added throughout to the barometric differences on account of the ratio of the diameters of the tube and cistern. The height of the column of mercury, in the upper and lower barometer, under equal pressure, was in all cases carefully examined, and the difference, if any, allowed as an index error to the lower barometer. I have great pleasure in remarking, that I found much less difficulty than I had anticipated, in getting corresponding observations made with the hygrometer, on the correctness of which I could sufficiently depend: the ingenuity in the principle of this instrument, and the simplicity of its application, together with the decisive nature of the results which it gives, independent of the labour, and, at best, the uncertainty of formulaic deduction, form its great advantage over the methods by evaporation, or the indications of hygroscopic substances: these particulars excite an interest in its trial, in persons to whom it was previously unknown, which is probably the reason that the distrust, which is almost always, in the first instance, expressed of precision in the observation itself, is found to give way in practice so much sooner than might be supposed. It may be useful also to travellers in warm climates, to add a remark from my own experience, that in ascending elevations, or in journeying inland over rough roads, the ether carries perfectly well in a bottle in the waistcoat-pocket, with a common cork capped with leather; and that the expenditure of ether altogether will probably fall much short of the estimate, as, with ordinary care, very little will be wasted."

[page] 327

Indications of Daniel's Hygrometer, in the Gulf of Guinea, between the 1st and the 15th of May.

May 1000. Air. Point of Deposition. Prepertion of Moioture in the Air to the
Quantity which would be required for the
Seturation at the exlating Temperature.
1 9½ A.M.
3 P.M.
6 P.M.
o
78.5
81
81
o
74.5
74.7
76
87.5 parts in 100.
2 6½ A.M.
2½ P.M.
6 P.M.
79.5
82
83
76
75
75
89.3
79.5
77.4
3 6 A.M.
3 P.M.
6 P.M.
79
83.5
82.5
76
77
77
90.7
81.3
83.9
4 6 A.M. 81.5 77 86.3
5 7 A.M.
2 P.M.
81.5
83.5
76
77
83.4
81.3
6 7 A.M.
2 P.M.
81.5
83.5
76
77
83.4
81.3
8 10 A.M. 80 76 87.6
9 3 P.M. 86 79 80.1
11 6 P.M. 83 80 90.9
12 P.M. 79 Rain 100
13 9 A.M. 80 76 87.6
15 7½ A.M. 82.5 78 86.6

"At St. Thomas's, between the 26th of May and the 12th of June, the range of the hygrometer, observed generally three times a day, in a house by the sea-side, with an extensive swamp behind it, was limited to 71° and 74.5°; the tempera-

[page] 328

ture of the air ranging from 74 and 75 at night, to 85 and 86 in the day; (the nights being cooled by the air descending from the high land of the interior;) the average moisture, relatively to that of the Gulf of Guinea, generally, and away from the land, may be considered at a mean between the extremes corresponding to the observation above mentioned; i. e., 79,6, to 85.1.

On passage between the River Gaborn, on the coast of Africa, under the equator, and the Island of Ascension, in 7° south latitude, and midway between Africa and America, I obtained the following results:—

June. Time. Air. Surface Water. Point of Deposition. REMARKS.
17 8 A.M. 74.5 o
73
o
69
Roll's Isl.E.N.E.
18 8 A.M. 74 74.2 69 Wind during the passage fresh from s. to s.s.w. The ship passed from the equinoctial current into its north westerly feeder on the morning of the 22nd when the surface water resumed the ordinary ocean temperature.
19 8 A.M. 73.5 72.5 68.5
20 8 A.M. 73.5 72.8 67.5
21 8 A.M. 74 77.5 66.5
22 8 A.M.
77
75.5
77.5
77.3
69
68.5
23 8 A.M. 75.5 78.2 67.5
24 8 A.M.
4 A.M.
76
77
78.2
78.2
66.5
69.5
25 7 A.M. 76.5 76.5 Close in with Ascension.

During ten days' residence at Ascension, the thermometer, sheltered, varied between 75° and 84° during the 24 hours. The point of deposition

[page] 329

from 65°, before sunrise, to 70.5 once, and generally to 69 and 70 in the afternoon. The details of a barometrical measurement of the height of Ascension have been given above.

Bahia.—Place of observation, a plateau unsheltered, between the consul's house, and the edge of a cliff 213 feet above the sea, facing the harbour of St. Salvador.

The following Table contains the observations for the mean temperature of the air, made with a transparent register thermometer, suspended in a polished copper cylinder, with holes at the top and bottom, in a fair exposure to the wind, but sheltered from the heavens, eight feet above the ground:—

[page] 330

Days. Time. Temperature of the Air. Mean. REMARKS.
July 24 and 25 Sun-set to sun-rise
Sun-set to sun-rise
°
76.
82.
°
69.5
75.5
°
75.75
– 25 and 26 Sun-set and midnight
Midnight and sun-rise
Sou-rise and noon
Noon arid sun-set
75.5
71.5
80.5
82.
71.
68.
67.5
76.
74 It was almost constantly cal and clear, until the 28th, when southerly winds set in with frequent rain–the air become drier, and the temperature less
– 26 and 27 Sun-set to midnight
Midnight to sun-rise
Suu-rise to noon
Noon to sun-set
76.
72.
83.
82.
72.
73.5
73.
73.
75.5
– 27 and 28 Sun-set to midnight
Midnight to sun-rise
Sun-rise to noon
Noon to sun-set
73.
73.
78.
77.
70.
68.5
69.
71.
72.5
–23 and 29 Sun-set to midnight
Midnight to sun-rise
Sun-rise to noon
Noon to sun-set
72.
70.5
78.
77.
67.
64.
71.
73.
71.5
–29 and 30 Sun-set to midnight
Midnight to sun-rise
Sun-rise to noon
Noon to sun-set
73.
72.
80.
80.
71.5
67.
69.5
73.
73. Southerly wind, with frequent rain.
–30 and 31 Sun-set to midnight
Midnight to sun-rise
73.5
72.
68.5
65.

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"From the twenty-third to the twenty-seventh, the point of deposition varied in the day, i. e., from six A.M., to six P.M., between 66°, and 70.5°, being on each day either 70° or 70.5° in the afternoon.— After the southerly winds set in, on the twenty-eighth, with occasional rain, the point of deposition varied from 61° to 64°, at the same hours, and continued so until the end of the month, except, of course, when the rain was actually falling.

On the passage along the Brazil coast, between Bahia and Pernambuco, generally in sight of land, the temperature varied between 74.4° and 78.4°, the point of deposition between 66.5° and 70.3°, and the surface water between 77.1° and 78°, all being observed at three periods of the day, before eight, after noon, and before sun-set.

Between Pernambuco and Maranham, also along the Brazil coast, temperature 76.5° to 78.5°, surface 77.8° to 78.2°, and the point of deposition from 70° to 74°.

At Maranham we inhabited a house in the middle of the city, without a garden or court; consequently, we observed only the hygrometer and a thermometer in our upper room in the house, open in every direction, and surrounded by a gallery and viranda; the thermometer, at midnight, was generally 76° or 77°. At six A.M., 80°, at noon 84°, and at sun-set 82°. The point of deposition very regularly 71° at midnight, and rising to 73° in the day, being only once observed so low as 69°.

Between Maranham and Trinidad, the air, during the day, from 79° to 83°, the point of deposition from 73° to 75.5°—the surface water from 83° to 84°.

[page] 332

Port Spain, Trinidad.—A transparent register thermometer was suspended in a copper cylinder, pierced with holes at the top and bottom, in the centre of the belfry, in the tower of the new Protestant church, open to the wind on the four sides, by windows fitted with Venetian shutters. The pavement of the church was eighteen feet above the level of the sea: the maximum and minimum of the thermometer were registered each day at noon.

Date. Air. Mean.
Sept. 23 and 24 °
74.
°
88.
°
81.
24 and 25 74. 86. 80.
25 and 26 73.5 86. 79.75
26 and 27 74.75 86.25 80.5
27 and 28 74. 84.5 79.5
28 and 29 73. 85 79.
29 and 30 75. 87.5 81.25
30 and 1 Oct. 74. 85.5 79.97
1 and 2 72. 88. 80.
80.11 Lat. 10oN.

The sun had passed the corresponding declination, and had arrived at the equator.

Between Trinidad and Jamaica, in the run across the Caribbean sea, the following were the observations:—

[page] 333

Date. Time. Air. Point of Deposition. OBSERVATIONS.
Oct. 11 8 A.M. 83°.2 77°.5 Clouded, strong breeze.
2½ P.M 83. 78.5 Clouded, less wind.
12 8 A.M. 82. 76.5 Clear sunshine, fresh breeze.
13 8 A.M. 83. 77.5 Sunshine, with clouds.
14 8 A.M. 82. 78. Sunshine, with clouds.
15 8 A.M. 82. rain Heavily clouded.
16 8 A.M. 83.4 77.5 Sunshine and fine.
17 8 A.M. 82. 78. Fine.
Noon. 82. 77.

Port Royal, Jamaica.

For the Mean Temperature.—A register thermometer was suspended in a copper cylinder, pierced with holes in the top and bottom, in a free current of air, in Fort Charles, sheltered by a platform overhead, and exposed to the draft of the sea breeze through an embrazure, in which the thermometer was placed. It was 200 feet distant from the windward shore of Port Royal, and the height above the sea, eight feet.

Date. Extremes Registered. Mean. OBSERVATION.
Oct. 23 to 24 83°.5 77°. 80°.25 Weather continually clouds, and esteemed unusually cold.
24 to 25 84.5 78. 81.25
25 to 26 86. 76. 81.
26 to 27 87. 76. 81.5
27 to 28 86. 76. 81.
28 to 29 86.5 76. 81.25
29 to 30 87.5 76.5 81.5
Nov. 4 to 5 86. 76. 81.
81.17

[page] 334

Hygometry.—Between the twenty-first October, and the thirtieth, the maximum of the point of deposition varied on different days, between 76° and 78°."

The particulars of a measurement of the mountains of Jamaica have been already given.

[page 335]

METEOROLOGICAL OBSERVATIONS

IN BRAZIL, AND ON THE EQUATOR.

By ALEXANDER CALDCLEUGH, ESQ.

I AM indebted to the obliging attention of Mr. Caldcleugh for the following interesting particulars of the climate of Brazil, and observations in the northern and southern trade winds:—

"The summer at Rio de Janeiro begins about the months of October or November, and lasts until March or April. This is the wet season, but the rains by no means descend from morning till night, as in some other tropical countries, but commence, generally, every afternoon, about four or five o'clock with a thunderstorm. The heaviness of the rain can only be conceived by those who have been in these latitudes. This fall naturally arrests the sea-breeze, and the succeeding night is dark and cloudy. Formerly these diurnal rains came on with such regularity, that it was usual, in forming parties of pleasure, to arrange whether they should take place before or after the storm. During this period of the year there is seldom, if ever, a deposition of dew.

From April until September very little rain falls: vegetation almost stops, and to the eye of every one who has not just arrived from Europe, a

[page] 336

wintery appearance is discernible. The land and sea breezes do not succeed each other with the same regularity, and are, besides, more frequently disturbed by violent gusts from the S.w imagined to be the tails of those destructive winds, the Pamperos of the River Plate. The nights are beautifully clear; Venus casts a shadow, and the southern constellations are seen in all their beauty. The dews, as might be expected, are at this season very copious. The annual mean height of the barometer in Rio da Janairo is about 30.275, and of the thermometer a fraction above 73° Fahrenheit.

The particular observations upon the climate which I was enabled to make during my residence in the country, are contained in the following journal:—

[page 337]

MR. CALDCLBUOH'S Meteorological Journal, commencing at Rio Janeiro, 1st August, 1821, and continued on the Route to Villa Rica.

Z

[page] 338

Day of Month. Locality. Face of the Country. Barometer noon.
Aug. 1 Rio de Janeiro The Country about Rio abounds in cone-sgaped hills, colthed to their summits with wood. The state of the Barometer was not recored during this period.
2 "
3 "
4 "
5 "
6 "
7 "
8 "
9 "
10 "
11 "
12 "
13 "
14 "
15 "
16 "
17 "
18 "
19 "
20 "
21 "
22 "
23 "
24 "
25 "
26 "
27 "
28 "

[page] 339

Difference. Weight Vapour in Grains in Cubic foot. REMARKS.
Observation at 12 o'Clock, when not otherwise expre sed.
11 5.560 Fine.
11½ 5.467 Fine.
9 6.029 Fine, a little cloudy.
11 5.393 Fine.
14 4.939 Fine, regular breezes.
17 4.756 Fine.
16 5,080 Fine.
17 4,909 Fine.
15 5.089 Fine.
17½ 4.756 Fine.
17 4.756 Fine.
16½ 4.918 Fine.
15 5.251 Fine.
16½ 5.980 Fine.
14 5.434 Fine.
15 5.435 Fine.
16 5.242 Fine.
13 5.625 Fine.
13 5.982 Fine–Lete lea breeze.
11 6.184 Fine.
141 5.616 Fine.
8 7.430 Late sea breeze–Rain at night.
7 6.834 Fine.
10 6.387 Fine.
10 6.493 Fine.
8 6.909 Fine.
5 8.715 Warm, heavy rain at night.

Z 2

[page] 340

Day of the Month. Locality. Face of the Country. Barometer noon. And.
Aug. 29 Rio de Jaueiro
30 Left Rio de Janeiro 29.753 75
31 Porto d'Estrella Swampy. 29.801 75
Sept. 1 Mandioca 29.654 71
" Pass of the Organ Mountains. Exlremely mountainous thicky wooded. 27.152 67
2 Padre Contêa 27.752 66
3 Pampulha 28.152 68½
4 There leagues in advance 29.250 68
" Over the Valley of Paraiba 28.202 80½
" Bank of the Paraibuna 29.902 78
5 " 29.150 78
6 Mathias Barbosa 27.650 74
" Morro de Mideuras 27.650 74
7 Alcaide Môr 27.701 65
" Chapeo d'Uvas 27.600 74
8 " 27.751 64
9 Montiqueira 27.351 59
" Height of the Serra 26.601 64½
10 Barbacena Open plain with little timber but the pine. 26.552 50
11 Four leagues in advance
12 Queluz Villa 27.003 60½
13 Congonha do Campo 27.302 59½
14 Capao d'Olanda Topaze Mine Very mountainous country with little wood. 26.250 65
15 Villa Rica 26.370 64
16 " 26.400 65
17 " 26.420 74
18 Marian 27.951 68
" Itacolumi of Marianna 26.254 70½

[page] 341

Difference. Weight Vapour io Graine to Cubic foet. REMARKS.
Observation at 12 o'Clock, when set otherwise expressed.
6 7.233 Rainy in squalls.
11 6.482 Fine, but sultry and cloudy.
13 5.982 Gloomy, heavy rain at night.
10 5.660 Fine, thunderstorm.
7 5.877 Slight rain, l-80th.
8 5.156 Gloomy, 7 A.M.
9 5.303 Gloomy morning.
12 4.664 Fine, most oppressive sun.
5 9.018 Noon observation.
8.184 Evening—Temperature of the river 71°.
10 7.183 Morning cool and gloomy.
8 5.496 Slight rain, which made the track slippery.
5 6.945 Clear about one o'clock.
5.710 Dull and cold morning.
7.221 Fine evening.
4 5.908 Cloudy, afterwards heavy rain.
6 4.420 Severely cold morning, mist and rain.
5 5.525 About eleven A. M. foggy.
2 4.179 Morning severely cold.
Felt the cold severely.–From a variety of causes prevented examining the Barometer or Hygrometer.
4.120 Thick fog, afterwards oppressive sun.
10 3.988 Clear morning.
12 4.852 Bright morning.
9 4.689 Fine.
8 5.156 Fine—Observation made in the morning.
7 7.221 Fine, idem at noon.
5 6.072 Fine.
6.933 At one P.M.

[page] 342

Day of the Month. Locality. Face of the Country. Barometer noon. Attd.
Sept. 18 Maynerte very mountainous country with little wood 27.950 79
19 " 28.208 68
20 Mangeleguas 27.802 75
21 Bandeira 27.601 68
22 'illa Rica 26.394 69
23 " 26.380 73
24 " 26.412 72
25 " 26.392 71
26 " 26.361 69
27 " 26.376 70
28 " 26.381 70
29 " 26.400 73
30 " 26.390 73
Oct.1 " 26.416 72
2 " 26.382 70
3 Coxo de Agua Abounding in wood. The barometer was left at Villa Rica during this excursion.
4 Congonha de Sabará
5 Sabará
6 Caete
7 St. Joao
8 Inficionado
9 Vamos vamos
10 Villa Rica 26.378 67
11 " 26.408 66
12 " 26.400 67
13 " 26.398 68
14 " 26.399 68½
15 " 26.406 70

[page] 343

Row Eligible Difference. Weight Vapour is Grains in Cubic foot. REMARKS.
Observation at 12 o'Clock, when set otherwise expressed.
8 7.208 In the evening.
4 6.567 Thick fog.
4 7.715 Very thick fog.
4 6.265 Cloudy morning—Torrents of rain in the afternoon.
2 6.988 Gloomy. Noon observation.
3 7.728 Fine.
3 7.495 Very foggy.
3 7.277 Clearer.
4 6.567 Cloudy.
5 6.556 Sun obscured.
6.863 Cloudy, light rain.
6 7.233 Morning foggy, afterwards clear.
4 7.482 Fine, a little foggy.
7.277 Cloudy and thick mist.
6.988 Thick mist–Dreadful storm.
4 8.746 At two P. M. great thunderstorm.
3 10.536 At three P. M. idem.
2 12.210 Very warm–Violent storm at night.
4 8.746 Storm of thunder and rain at four P. M.
6 6.544 Fine.
5 6.556 Foggy.
7 6.427 Foggy.
3 6.471 Thick mist.
4 6.082 Idem.
4 6.265 Idem.
8 6.587 Idem.
6 6.061 Idem.
6.556 Idem.

[page] 344

"On the 15th October I began to retrace my steps to Rio de Janeiro. Having left the barometer at Villa Rica, I made no kind of observations cm the weather. The rains having commenced, the roads were in some places in almost an impassable state, and I scarcely think the barometer could have escaped, from the many falls my mule experienced.

I had imagined that the great humidity at Rio proceeded from the saline particles blown over by the sea breeze; but, on examining the foregoing register, it will be remarked, that there was more vapour in the air, on the 19th and 23th of August, before the sea breeze had commenced, than on the preceding days. I have no doubt, therefore, that when the land breeze prevails all day, which sometimes, though fortunately rarely, happens, the most vapour is contained in the air; and it seems to me this must be the case.

Beyond the Serra de Montigueira, the track leaves the mountainous and thickly-wooded country, and crosses a high table land, where the pine is the only tree that seems to flourish. The height of this, from the average of the barometrical observations above the sea, may be about 3720 English feet. Baron Humboldt gives the lower limit of the Mexican Pine (19 N. Lat.) at 1150 metres = 3769 English feet. I have seen this species growing in still lower situations in Brazil, but certainly with not so much luxuriance.

The means of the observations made at Villa Rica, are as follows:—

Bar. 26.393—attached and detached thermome-

[page] 345

ter 69½°—dew point 65°, and grains of vapour in cubic foot 6.577. Its consequent height above the sea 3969 feet.

The mean quantity of vapour, observed by Mr. Danieli, for the two years ending with the summer of 1821, was, grains 3.652, not much more than half the mean at Villa Rica. The prevailing winds there were south and south-east.

I remarked invariably the barometer to stand lower, and the quantity of vapour more considerable, in the evening than the following morning. When overlooking some of the thick woods, it was curious to see the warm vapour ascending like smoke from particular spots, where the foliage did not form a mechanical obstruction.

On the excursion made from Villa Rica to Sabarà, it will be seen that violent thunder-storms were, experienced almost daily: nothing causes so much attention to be paid to the weather as being exposed to its changes; and I could not: help noticing the way these storms commenced. The sky was perfectly clear until about two or three o'clock, when some light white clouds were seen approximating the sun with great rapidity. Sometimes they all passed, but if one lingered, as if within its influence, thunder was heard, and in a few minutes no remains of a blue sky were visible. The storm commenced directly, and the change that took place in the temperature often caused a kind of whirlwind.

As after all, perhaps, we must search for the cause of that singular excrescence, the goitre, or

[page] 346

wen, in the state of the air or vicissitudes of climate, it may not be irrelevant to mention that I met by far the greater number of persons afflicted with this complaint near Sabarà.

From the degree of cold in the province of the mines, the hue of the negroes is much deeper than in Rio de Janeiro. The Mineiros who come down, complain much of the heat, and have their health affected. This may proceed, however, from other causes, such as excess in fruits, which are unknown in their province, and a mode of life entirely different I am inclined to think, on the whole, that foreigners would consider the coast more healthy than the interior.

Having reached Rio de Janeiro, I embarked on board His Majesty's ship Owen Glendower, for England, on the 22d November, and as soon as the usual sickness had abated, recommenced my observations with the hygrometer. The Hon. Captain Spencer, whose only aim seemed that of rendering all under his command and on board his ship perfectly happy, and in which it is almost superfluous to say he was most successful, dedicated a portion of his time to, and took considerable interest in, these experiments. Many of them recorded here were conducted by him, and he indeed suggested an improvement in the instrument, (that of colouring the bulb,) which on our arrival in England we found had been already contrived.

The hygrometer was accidentally broken on the 27th December, and, being provided with only one,

[page] 347

my observations ceased on that day. When I commenced using the instrument, I was almost afraid to touch it, from its apparent delicacy, but was soon convinced, from the many rude shocks it underwent, that it was stronger than I had imagined; more than common carelessness, indeed, is required to break it. I may be permitted to add, that I think no traveller will find any inconvenience from carrying this hygrometer, or its accompaniment, a small stock of ether; the latter I usually placed among my linen.

Although the observations, of necessity, ended here, I had the gratification of thinking they were continued through the south-east trade, south of the equator, and until we were on the northern limit of the north-east trade, which it is well known prevails on the northern side. On examining the register it will appear, that on the days when the trades were fresher, there was a slight diminution of vapour; and that, as we approached near the equator, we approximated the point of saturation, the precise position of which, probably, varies according to the longitude and season, as is the case with the trades themselves.

In these winds there is something so exhilarating, that one with difficulty believes so much vapour exists as the hygrometer indicates. Baron Humboldt, who did not proceed farther south than ten degrees north latitude in crossing the Atlantic, marks eighty-six degrees of Saussure's instrument in that altitude.

A set of experiments conducted on board some

[page] 348

of the foreign ships that endeavour to pass the line in improper longitudes, and consequently experience calms of many days' duration with much rain, would prove particularly interesting."

[page break]

[page break]

METEOR Captain the Hon. R. C. SPENCER,
Time of the day. Latitudde S. Ther. Daw
Point.
Difference Grains of
Vapour in
Cubic Foot.
Time of the day
when Hygrometer
was used.
8 A.M.
noon
8 P.M.
°
24.25
76
76
73
72
3
4
8.488
8.212
1 P.M.
6 P.M.
8 A.M.
noon
8 P.M.
24.17 76
76
70
70
6
6
7.688
7.688
10 A.M.
3.3 P.M.
8 A.M.
noon
8 P.M.
23.25 74
76
69
72
5
5
7.469
8.198
9 A.M.
4 P.M.
8 A.M.
noon
8 P.M.
22.31 76½
76½
71
70

7.951
7.689
9 A.M.
3 P.M.
8 A.M.
noon
8 P.M.
20.57 76
76
70
72
6
4
7.640
8.213
9 A.M.
3 P.M.
8 A.M.
noon
8 P.M.
18.52 77
79
71
70
6
9
7.937
7.676
6 A.M.
3 P.M.
8 A.M.
noon
8 P.M.
15.37 76
76
72
70
4
6
8.213
7.689
9 A.M.
3 P.M.
8 A.M.
noon
8 P.M.
11.54 76
77
70
71
6
6
7.689
7.937
8½ A.M.
3 P.M.
8 A.M.
noon
8 P.M.
8.6 78
77
72
71
6
6
8.185
7.937
8½ A.M.
3 P.M.
8 A.M.
noon
8 P.M.
4.55 79
79½
74
73
5
8.732
8.438
9 A.M.
3 P.M.
8 A.M.
noon
8 P.M.
2.14 80
79
75
74
5
5
9.013
8.732
8¾ A.M.
3 P.M.
8 A.M.
noon
8 P.M.
0.8N 79½
79½
76
76

9.334
9.334
8.50 A.M.
3 P.M.
8 A.M.
noon
8 P.M.
2.14 80
82
78
79
2
3
9.958
10.263
8¾ A.M.
3½ P.M.

(To face Page 348).

[page 349]

REMARKS

UPON THE BAROMETER AND THERMOMETER,

AND THE

MODE OF USING METEOROLOGICAL
INSTRUMENTS IN GENERAL.

BY offering the following remarks upon meteorological instruments, I would not wish it to be supposed that I claim, for the observations which I have hitherto recorded, a greater degree of precision than attention to the usual precautions has been sufficient to confer: but in the course of my experiments, the necessity of much greater care and method has become strongly impressed upon my mind, and I think that it may not be wholly without its use, to indicate such measures as the result of my experience suggest, as likely to ensure that degree of perfection, of which the science of meteorology is doubtless susceptible. I have little of novelty to offer upon the subject; but if, by repeating well-known observations, I can contribute to excite that attention to them which is absolutely necessary to success; if the numerous observers of atmospheric phenomena may possibly be thus engaged to that strict co-operation, which alone can prevent their daily labours from proving abortive, a great and important object will be attained.

Much of my attention has lately been given to the manufacture of barometers. The committee of

[page] 350

the Royal Society, appointed to take into consideration the state of the meteorological instruments, did me the honour to request that I would attend to the construction of a new barometer for their apartments; and as, in the course of the dose attention which I paid to the most minute details, I had occasion to make many practical remarks, I cannot, I think, do better than here introduce the account of the process which I had prepared for the society.

In the course of the experiments I was led to a new method of filling the tube, which I flatter myself may prove generally useful, and tend, by the facilities which it affords, to the perfection of the instrument

Previous to commencing the operation, some ex-experiments were undertaken to ascertain the practicability and effect of introducing the metal, after the air had been abstracted, as nearly as possible, by means of an air-pump, and the mercury and interior surface had been exposed to the desiccating influence of a large surface of sulphuric acid. For this purpose a barometer tube was fitted with a stop-cock, which was screwed into the under surface of a pump-plate; on the upper surface stood a glass dish, perforated in the centre, and containing the acid. In this was placed a stand with glass legs, which received a funnel, the stem of which being drawn out into a capillary tube, passed down into the mouth of a small paper cone, resting upon the tube. The aperture at the upper part of the stem was closed by an iron plug, ground to fit, between which and the capillary opening was placed some cotton. The glass funnel was filled with clean mercury, carefully boiled, and many times filtered, and the whole was covered

[page] 351

with a glass receiver. Through a collar of leather, in the upper part of the receiver, passed an iron rod, which moved freely up and down, and fitted into a screw in the plug before mentioned, by which means it could be drawn up, and re-placed, at pleasure. The apparatus being thus arranged, the pump was worked, and the air exhausted from the receiver and tube. Air was at first given off from the surface of the acid in abundance, and a few bubbles passed up from between the mercury and the glass; but none appeared upon the surface of the mercury. When the rarefaction had been carried as far as possible, the siphon-gauge stood at about half an inch. The iron plug was carefully withdrawn, and the mercury began to trickle very gradually into the tube. In its fall it was broken into small globules, many of which adhered to the sides of the glass; and, notwithstanding the utmost precaution and frequent repetitions of the experiment, the column of mercury, as it rose, contained very minute cavities, which decreased in size as the weight increased; and when the pressure of the atmosphere was restored, were only discernible upon very close examination. When the air was again extracted, they returned to their former size, and again diminished upon its restoration. The difficulty of getting rid of these cavities appears to me to arise chiefly from their form: for the mercury, assuming the shape and properties of a dome round the bubble, resists a degree of pressure which would otherwise cause it to run together.

To avoid this mechanical motion of the fall of the mercury, the apparatus was varied as follows: A

[page] 352

small tube was passed down to the bottom of the barometer-tube, and was fastened at the top by a piece of cork, to prevent its coming in contact with the sides. The lower aperture had been lessened, and the small paper funnel was inserted into the upper end. The exhaustion having been made as before, the mercury was allowed to trickle down the interior of the inner small tube, from the bottom of which it issued slowly; and gradually rose in the larger, in perfect and uninterrupted contact with the glass. When the tube was full, the air was let into the receiver, and the tube detached from the plate. To prevent the possibility of the disengagement of any particles of air, which might be entangled in the mercury of the small tube, its orifice was hermetically sealed by a lamp, and the tube itself full of mercury, carefully withdrawn from the large one. The closest examination, with a magnifying glass, of the barometer-tube so filled, failed to detect the minutest air-bubble, and the surface everywhere was as resplendent as that of the most perfect mirror. The application of heat produced no alteration in this appearance, nor any traces of either air or moisture. The small tube, upon inspection, was found to contain very minute, and scarcely visible, specks, like those of the tubes filled in the first method; but these were, of course, diminished in quantity, in proportion to the diminution of the tube in which they were formed.

The success of this experiment was so great, that, in any common case, it would scarcely have been thought necessary to subject the barometer to the troublesome and hazardous process of boiling

[page] 353

the mercury; but upon this occasion it was resolved, that no possible precaution should be omitted.

The tube, which was selected for the society's barometer is 33¾ inches long, its exterior diameter 0.86 inch, and the diameter of its bore 0.530 inch.

These measures were taken at the upper extremity, and it is very regular for 14 inches, but enlarges a little from that point downwards. It is ground flat at the lower end.

Many tubes were destroyed, after all the trouble bestowed upon their mensuration and filling, by the after-process of boiling, which, in tubes of such large capacity, was found to be very troublesome and hazardous, and required the glass to be of a red-heat. The above dimensions are those of the barometer now complete.

The cistern is turned in well-seasoned mahogany, and there is a small cavity in its bottom to receive the end of the tube, which rests upon it: a groove communicates with the cavity, to ensure the free passage of the mercury. By means of the float in front, the level may be very accurately taken. Fifty inches, measured in the upper part of the tube before it was sealed, in four equal proportions, raised the float exactly half an inch; the correction, therefore, for the capacity of the cistern, is 1/100th.

The cistern being accurately levelled, and the tube and thermometer both in their places, the quantity of mercury was adjusted to the upper edge of the black line on the stem of the float: a card gauge of nearly the diameter of the cistern was then fitted to slide upon the tube, which was fixed perpendicularly in its place. The lower edge of the

2 A

[page] 354

gauge was made to coincide with the surface of the mercury on both sides, and at its contact with the glass, two distinct marks were scratched upon the tube. From these marks, twenty-nine inches were measured off by a brass dividing engine, which was formerly the property of the late Mr. Cavendish, and at that distance, another distinct mark was made.— The utmost care was taken to read off these distances by means of lenses, and the temperature of the scale, the glass, and the mercury, was 54°.

It being deemed too hazardous an experiment to attempt to boil, at once, so large a body of mercury as would be contained in a tube of this capacity, it was resolved to perform the operation in two partions, and under the diminished pressure produced by an air-pump. Accordingly, seventeen inches of mercury were introduced into the tube with all the precautions above described. During the exhaustion, no air was disengaged from the mercury, care having been taken to fill the funnel without agitation. The contact with the glass, while the siphon gauge of the pump stood at .4 in., was perfect, and the appearance of the tube when detached as bright and compact as could be wished. The air was again exhausted, and by means of a stop-cock, the vacuum preserved. The tube was then gradually heated before a fire, and the boiling, afterwards, cautiously begun over a large spirit lamp. The upper part of the column was first strongly heated, and when it had arrived at the point of ebullition, the boiling was slowly continued downwards. When it had reached the bottom, it was again as gradually conducted to the top. The bubbles of vapour freely

[page] 355

passed, with the assistance of a slight degree of agitation, from one end of the column to the other; and very bright flashes of green light accompanied their extrication. One minute globule of air alone was detected during the heating, notwithstanding the diminished pressure,: aad this was readily extricated; and there was not the slightest condensation of moisture visible in the cold portion of the tube.

The cooling was conducted with all the precautions used in healing; and to allow the mercury to resume the temperature of the air, the completion of the process was deferred till the next day. After fifteen hours' repose, upon opening the stop-cock, the exhaustion was found to have been perfectly maintained: the apparatus was again arranged, the small tube being made just to touch the surface of the mercury in the larger. A quantity of mercury was then introduced as before, which was found after the interior tube had been withdraw, tp amount to twelve inches and a half. The appearance was as perfect as in the first operation, except that two or three wry minute specks appeared at the junction of the two portions. These scarcely-visible air-bubbles had probably been introduced in extracting a very small particle of cement, which had Men down; the rod with which this was done having been passed about 1/10 th of an inch below the surface of the mercury. They disappeared under atmospheric pressure. The whole length of the mercurial column was now twenty-nine indies and a half, leaving only three inches and a half of the tube unoccupied, which was deemed but barely sufficient to prevent the communication of the heat

2 A 2

[page] 356

melting the cement and destroying the exhaustion. When the air was abstracted, the junction was again just discernible. The boiling was begun as before from the top, and carried downwards to about two inches below the union of the two portions. The little air-bubbles were visibly expanded and easily passed up: the boiling was continued for a considerable time, and large bubbles of mercurial vapour, accompanied with bright green light, freely traversed the whole column. The tube was then suffered gradually to cool. Its appearance was compact and bright: a very slight haziness or discoloration was observable at the junction, but not the slightest indication of air even under exhaustion. No precipitation of moisture was perceptible in the cool portion of the tube.

It was the original intention to have completed the filling of the barometer, by boiling the remaining three inches and a half; but, upon consideration of all the circumstances, and, especially of the necessity there would be of performing this under atmospheric pressure, it was concluded not again to expose the tube to so much risk. The column already boiled comes within the range of the atmospheric oscillations, and the utmost care was taken in filling the remainder, as before, in vacuo. The last portions of mercury were introduced hot, and the whole was left for forty-eight hours, to take the temperature of the air. The tube was then carefully inverted in the cistern; but the mercury, notwithstanding its great body, did not descend till after it had received two or three smart concussions. This, I believe, to be the most certain proof of the

[page] 357

complete displacement of every particle of air. The adjustments, of the scale, with its nonius to the upper mark upon the tube, and of the quantity of mercury in the cistern to the line upon the float, were now easily made, and the instrument was fixed in its proper situation. Whenever the mercury vibrates in the tube, a beautiful green light flashes through the vacuum, and the crackling sound of electric excitation is heard when the finger is presented to it. Electric attraction and repulsion are also exhibited by presenting a piece of gold leaf to its influence.

Every thing has been studied in this instrument to render accuracy attainable, with as little trouble as possible to the observer. The diameter of the tube renders the correction for capillary action, almost unnecessary—the correction for the capacity of the cistern has been contrived to be 1/100 th of the result above or below the neutral point, 30.576 — and a scale is engraved upon the front, of the correction to be applied for the expansion of mercury and mean dilatation of glass; by which the observation may be at once reduced to the standard temperature of 32°. A small thermometer in front of the instrument dips into the mercury of the cistern. The specific gravity of the mercury employed was carefully ascertained at the Royal Institution, by Mr. Faraday. The temperature of the metal and the water were both 40°, and 1000 grains of the former displaced 73.4 grains of the latter: hence 1000/73 = 13.624.

One of my chief objects during these experiments has been to ascertain the agreement of different ba-

[page] 358

rometers made with equal care and independently graduated, after all the necessary corrections have been made for accidental differences. I, therefore, attended particularly to the construction of a mountain barometer for my own use, which was filled in vacuo, and afterwards boiled. After the process, the tube was perfect in appearance, the mercury adhered when reversed, and the electric light was very visible. The graduation was made with every care from the surface. The interior diameter of the tube is 0.15 in., and the correction for the capacity of the cistern 1/41 — the neutral point 30.180;—I shall here give the details of three separate comparisons of these two instruments.

Reyal Society's Barometer. Mountain Barometer.
30.576 Temp. of Mer. 50°. 30.526 Temp. of Mer. 50°.
–.047 Correction for Temp. –.047 Correction for Temp.
30.529 30.479
+.006 Capillary Action. +.088 Capillary Action.
30.535 30.567
+.009 Capacity of Cistern. 30.535 30.576
29.872 Temp. of Mer. 64°. 29.849 Temp. of Mer. 70°.
–.007 Capacity of Cistern. –.008 Capacity of Cistern.
29.865 29.841
–.082 Temp. of Mer. –.098 Temp, of Mer.
29.783 29.743
+.006 Capillary Action. +.088 Capillary Action.
29.789 29.831

[page] 359

Royal Society's Barometer. Mountaia Barometer.
29.756 Temp. of Mer. 63°. 29.742 Temp. of Mer. 72°.
–.008 Capacity of Cistern. –.010 Capacity of Cistern.
–.047 Correction for Temp. –.047 Correction for Temp.
29.748 29.732
–.078 Temp. of Mer. –.102 Temp. of Mer.
29.670 29.630
+.006 Capillary Action. +.088 Capillary Action.
29.676 29.718

The results of these comparisons disappointed me at first, as I had been induced to expect a much nearer accordance, after all the pains that had been taken. Upon reflection, however, I am inclined to think that the apparent discordance is in favour of the instruments, and that the difference points to an error in one of the corrections which has been overlooked. In the first place, it will be remarked that the difference .040 in. is constant, and its cause, therefore, is probably to be sought in the only constant correction, namely, that for capillary action. The quantities allowed have been taken from Dr. Young's Table* of the depression of mercury in barometer tubes, which was calculated from experiments. But these experiments were made with tubes in which the mercury had not been previously boiled, and a little consideration will be sufficient to shew that the results must have been very much influenced by this circumstance.

The phenomena of capillary depression depend upon the balance of the attraction of the particles of the fluid for each other, and for the

* Young's Nat. Phil., vol. ii., p. 669.

[page] 360

solid of which the tube is composed. The attraction of mercury for glass is well known to increase as the contact becomes more perfect; and, indeed, all the phenomena attending the boiling of a barometer-tube prove that this is the case. The depression in a tube, from which the air has been thoroughly expelled, must therefore necessarily be less than in one which has been filled without this precaution. Professor Casbois, of Metz, long ago remarked, that the depression of mercury in tubes of glass depended upon the imperfection of the contact; and M. de Luc, speaking of the same fact, observes—" MM. Cassini de Thury et Le Monnier employèrent des tubes de difierens diamètres, et cependant ils ne trouvèrent les différences dont parle M. de Plantade que dans les tubes que n'avoient pas été chargés au feu."—De Luc, Recherches sur l'Atmo., tom. i. p. 95.

The comparison above made would seem to indicate, that the depression is decreased one half by boiling, and by diminishing the correction accordingly, the two instruments exactly agree. By a comparison of several others, this estimate is greatly confirmed; and I have lately had an opportunity of putting it to a decisive test. Captain Sabine, before his departure for the North Seas, requested me to assist at an examination of his barometers: two were of the mountain construction, with iron cisterns, by Newman; and one was a marine barometer, by Jones. They had all been independently graduated from the surface of the mercury, and boiled. The interior diameters of the two first were .15 inch, and the correction for the capacities

[page] 361

of the cisterns 1/54th. The diameter of the last .31 inch, and the correction for the capacity of the cistern 1/11th. The neutral points of the three were the same, viz., 30.400. The following is the comparison of the three instruments with the one which had been already compared with that of the Royal Society. The latter I shall call 1; the two other mountain barometers, 2 and 3, and the marine barometer, 4. The heights are the means of four observations, taken independently by different observers, who never differed more than .005 inch.

No. 1. No. 2. No. 3. No. 4.
30.1835 30.1730 30.1845 30.1937 Temperature of Mercury alike.
.000 -.0042 -.0039 -.0187 Capacity
30.1835 30.1688 30.1806 30.1750
+.0880 + 0880 +.0880 +.0280 Capillary Action, by Dr. Young.
30.2715 30.2568 30.2686 30.2030

SECOND COMPARISON.

30.1130 30.1045 30.1087 30.1202
-.0016 -.0054 -.0052 -.0254 capacity.
30.1114 30.0991 30.1035 30.0948
+.0880 +.0880 +.0880 +.0280 Capillary action
30.1994 30.1871 30.1915 30.1228

It will be observed, that the three mountain barometers agree very closely together, the greatest difference from their average height being .008 inch, while the difference of the marine barometer, from the same height, is, in the first comparison, .062 inch, and in the second .070 inch. If, however, we

[page] 362

substiute half the conection for the capillary tion in the of the Royal Society's the difference is decreassed one-half.

No 1. No 2. No 3. No 4.
30.1835 30.1638 30.1806 30.1750
+.0440 +.0440 +.0440 +.0440
30.1800
30.1114 30.0991 30.1035 30.0948
+.0440 +.0440 +.0440 +.0440
30.1554 30.1431 30.1475 30.1088

The remaining discrepancy I have some reason for believing, is in the measurement of the neutral point of the marine barometer.

Thus we see that the application of half the correction for capillary depression, derived from experiments upon unboiled tubes, is most applicable to boiled barometers; and from its application to tubes of the greatly differing-diameters, .53 inch, .31 inch, .15 inch, we may pretty safely conclude, that the proposition is universal.

The following Table gives the results of the experiments of Lord Charles Cavendish upon capillary depression, the correct calculation of the same by Dr. Young, and the probable amount in boiled tubes.:—

[page] 363

TABLE I. Correction to be applied to Barometers. for Capillary Action.

Diameter of Tube. Cavendish. Young. Amount in boiled tubes.
Inch. Inch. Inch.
.60 .005 .0045 .002
.50 .007 .0074 .003
.45 .0100 .005
.40 .015 .0139 .007
.35 .025 .0196 .010
.30 .036 .0280 .014
.25 .050 .0404 .020
.20 .067 .0589 .029
15 .092 .0880 .044
.10 .140 .1424 070

During my experiments upon the filling and boiling of the barometer-tubes, my attention was particularly directed to the assertion of Sir Humphry Davy, (Phil. Trans., 1822, p. 74;, that " there is great reason to believe that air exists in mercury, in the same invisible state as in water, that is, distributed through its pores;" and to the disheartening feet, (if proved), that absorption of air " may explain the difference of the heights of the mercury in different barometers; and seems to indicate the propriety of re-boiling the mercury in these instruments, after a certain lapse of time." It is with much diffidence that I am compelled to differ from the high authority of the President upon this interesting point: but there is one observation which I made, which, I think, nearly disproves the suppo-

[page] 364

sition. All fluids, which are known to absorb air into their pores, invariable emit it when the pressure of the atmosphere is removed; but, upon an attentive examination of large bodies of mercury, variously heated in the vacuum of an air-pump, I never saw a bubble of air given off from the surface of the metal. Air will rise from the contact of the mercury with the glass in which it is contained, in exact inverse proportion to the care with which it has been filled, but it never rises from the surface of the mercury alone. The difficulty of properly filling a barometer-tube, I attribute to the attraction between the glass and the air—not to that between the mercury and air; and I believe that air will insinuate itself a little way between the glass and the metal at the exposed end of a boiled tube, but that this cannot happen if the end be plunged in mercury; and, consequently, that no deterioration of barometers is to be apprehended from this cause. Such a deterioration, indeed, if it had existed, must, long ago, have been detected from the instruments themselves; for, although the register of the Royal Society is not in such a state as to enable any one to reason upon its conclusions, that of the Royal Observatory of Paris, and some others, must have disclosed the fact.

With respect to the method of filling a barometer-tube in vacuo, recommended above, I have little doubt that it is as accurate as the method of boiling, if performed with proper care; and it is infinitely less troublesome and hazardous. The electric light is as strong in the tube, and its appearance is in every way as perfect. There is,

[page] 365

however, one precaution which it is proper to take, viz., to boil about an inch of mercury in the lower end of the tube, as this will prevent that concussion of the metal in its fall, which, breaking it into globules, is apt to entangle any of the residual air. At all events it will be a great improvement upon the common method, which merely consists in passing a large bubble of air up and down the tube, to collect together the smaller particles which adhere to the glass.

Indeed it is high time that more attention should be paid to the construction of meteorological instruments in general. The generality of observers are little aware of the serious inaccuracies to which they are liable. In the shops of the best manufacturers and opticians I have observed that no two barometers agree; and the difference between the extremes will often amount to a quarter of an inch: and this, with all the deceptive appearance of accuracy, which a nonius, to read off to the five hundredth part of an inch, can give.

The common instruments are mere play-things, and are, by no means, applicable to observations in the present state of natural philosophy. The height of the mercury is never actually measured in them, but they are graduated one from another, and their errors are thus unavoidably perpetuated. Few of them have any adjustment for the change of level in the mercury of the cistern, and in still fewer is the adjustment perfect: no neutral. point is marked upon them, nor is the diameter of the bore of the tube ascertained: and in some the capacity of the cisterns is perpetually changing from the stretching

[page] 366

of a leathern bag, or from its hygrometric properties. Nor would I quarrel with the manufacture of such play-things; they are calculated to afford much amusement and instruction; but all I contend for is, that a person, who is disposed to devote his time, his fortune, and oftentimes is health, to the enlargement of the bounds of science, should not be liable to the disappointment of finding that he has wasted all, from the imperfection of those instruments, upon the goodness of which he conceived that he had good grounds to rely. The questions, now of interest to the science of meteorology, require the measurement of the five hundredth part of an inch in the mercurial column; and, notwithstanding the number of meteorological journals, which monthly and weekly contribute their expletive powers to the numerous magazines, journals, and gar zettes, there are few places, indeed, of which it can be said that the mean height of the barometer for the year has been ascertained to the tenth part of an inch. The answer of the manufacturer to these observations is, that he cannot afford the time to perfect such instruments. Nor can he, at the price which is commonly given; for few people are aware of the requisite labour and anxiety. But who would grudge the extra-remuneration for such pains? Not the man who is competent to avail himself of its application. Let the manufacture of play-things continue, but let there be also another class of instruments which may rival in accuracy those of the astronomer.

It will, no doubt, be a part of the plan of the Committee of the Royal Society, to establish a stand-

[page] 367

ard barometer, and to afford every facility of comparison with it; so that any person, for scientific purposes, may have an opportunity of verifying an instrument: and it is to be hoped that they may proceed one step further, and take measures for ascertaining the agreement of the instruments at all the principal observatories, not only in this country, but in other parts of the world.

Nor is it in the construction of barometers only that the meteorologist has to complain of that want of accuracy which is so essential to the progress of his science: the same carelessness attends the manufacture of the thermometer. Few people are aware that they are all, even those which bear the first makers' names, made by the Italian artists, who graduate them one from another, and never think of verifying the freezing and boiling points. The bulbs are all blown with the mouth, and very little attention is paid to the regularity of the tube. The register thermometers particularly, are shamefully deficient. Those of Six's construction are often filled with some saline solution instead of alcohol; and in the best, the spirit is not exposed long enough in vacuo, to disengage the air with which it is mixed. The consequence is, that it is liable to become liberated, and, of course, interferes with the results. The original directions of the inventor have also been departed from, as to the proportions of the different parts, and as to the construction of the indices.

Those upon Rutherford's plan are universally sealed with air in their upper parts, which acts as a spring against the expansion of the column. The

[page] 368

iron index of one is liable thereby to become oxidated, and adheres to the glass, when the mercury passes it, and it becomes entangled; while the spirit of the other being unavoidably mixed with air, when the pressure is decreased by cold it is disengaged. The air may be again dissolved by increasing the pressure before a fire, and passing the bubble backwards and forward, and, in a state of solution, it does not appear to interfere with the equability of the expansion. This, however, is not certain; and, at all events, it is liable to re-appear, and is very troublesome. These imperfections are, by no means, necessary consequences of the construction of the instruments, although the makers are very willing that they should be so considered; but it requires great care and attention to guard against them. The general mounting of the meteorological thermometers is exceptionable in every way; buried as they are in a thick mass of wood, and covered with a clumsy guard of brass, they can but very slowly follow the impressions of atmospheric temperature.

The establishment of a perfect standard thermometer, which shall be accessible to all who may wish to consult it, will also, doubtless, be another object of the Committee of the Royal Society.

With respect to the change in the freezing point, which takes place in time in the best thermometers, I have lately had an unexceptionable opportunity of confirming the assertions of the French and Italian philosophers. Mr. Jones has obligingly put into my hands two thermometers of the late Mr. Cavendish, which have evidently been constructed with much

[page] 369

care. The mercury in the balls of both flows freely into the tubes when reversed; and when suffered to fall sharply, strikes the ends with a metallic sound. The same click may be heard in the bulbs when it is permitted to fall back, and the cavity closes without the slightest speck. These indications of a well-boiled tube are rarely to be met with in the common thermometers of the present day. They are mounted upon common deal sticks, and the graduation, which is only continued for a few degrees about the freezing point, is engraved upon a small slip of brass. The degrees are very large, and they are distinctly divided into tenths. Each degree of No. 1, occupies a space of .208 inch, and of No. 2, .130 inch. The scratch upon the glass for the freezing point is very visible in both. It is difficult to say for what purpose they were originally made, but evidently for some experiments upon the freezing point of water; and if they had been expressly constructed to verify the present point, they could not have been better contrived for the purpose. The bulbs of both were plunged into pounded ice, in which they were left for half an hour, and the height of the mercury was carefully taken by two observers with the aid of magnifying glasses. The result of the examination was, that in No. 1. the freezing point upon the scale was 0.4 degree too low, and in No. 2, 0.35 degree. There can be little doubt, I think, that the right cause of the phenomenon has been assigned, viz., the change of form and capacity which the glass undergoes from the pressure of the atmosphere upon the vacuum of the tube.

2 B

[page] 370

But attention to the perfection of instruments will be all in vain, without a proper degree of care and system in making and recording the observations. Observers would render a much greater service to science by devoting less of their time to the actual inspection of their instruments, and more to applying the proper corrections. If the meteorologist plead want of leisure, instead of daily observations, let him record the atmospheric changes of every second or third day, but let what he does record be correct. The proper hours of the day for observation are indicated by the barometer: the maximum height of the mercurial column is about 9 A.M., the mean at 12, and the minimum at 3 P.M. If a person have time to make three observations in the day, these are the hours which he should select: if circumstances only allow of his observing twice, 9 A.M. and 9 P.M. are the proper intervals: if only once, noon is the time. These fortunately happen to be, probably, the most universally convenient hours that could have been selected. In national observatories, it would not be too much to expect, that observations at 3 A.M. should be added to the preceding. Even those who merely consult the barometer as a weather-glass, would find it an advantage to attend to these hours; for I have re-parked, that much the safest prognostications from this instrument may be derived from observing when the mercury is inclined to move contrary to its periodical course. If the column rise between 9 A.M. and 3 P.M., it indicates fine weather; if it fall from 3 to 9, rain may be expected.

But the meteorologist, who wishes to confer a real

[page] 371

benefit upon science by his labours, has a much more tedious duty to perform than this. After taking the height of the barometer at the prescribed times with all possible caution, he will take care to make the proper corrections of the observations. If his instrument be not furnished with a contrivance for adjusting the level of the mercury, he will correct it according to the relative capacities of the tube and cistern: he will add the proper quantity for capillary depression, according to the diameter of the tube: and he will then reduce the height .to what it would have been, if the mercury had been of the standard density at the temperature of 32°.

For the purpose of facilitating this last operation, I shall here subjoin a Table of the proper correction, calculated by Mr. Rice from the experiments of MM. Du Long and Petit, upon the expansion of mercury and mean dilatation of glass:—

2 B 2

[page] 372

TABLE II. Correction to be applied to Barometers for Expansion of Mercury and Mean Dilatation of Glass.

Temp. Inches.
28.
Inches.
28.5
Inches.
29.
Inches.
29.5
Inches.
30.
Inches.
30.5
Inches.
31.
Inches.
31.5
25 +.017 .017 .017 .018 .018 .018 .019 .019
30 +.005 .005 .005 .005 .005 .005 .005 .005
35 -.007 .007 .007 .008 .008 .008 .008 .008
40 –.019 .020 .020 .020 .021 .021 .021 .022
45 –.031 .032 .032 .033 .033 .034 .035 .036
50 –.043 .044 .045 .046 .046 .047 .048 .049
55 –.055 .056 .057 .058 .059 .060 .061 .062
60 –.067 .068 .069 .071 .072 .074 .075 .076
65 –.079 .081 .082 .083 .085 .086 .088 .089
70 –.091 .093 .094 .098 .098 .100 .101 .103
75 –.103 .105 .106 .109 .111 .114 .116 .118

[page] 373

By calculating the monthly means, the obaeror will give a still greater value to his co-operation.

Attention to these directions, in addition to the benefit which it would confer upon meteorology, would also facilitate the purposes of barometric levelling; in return for which, the detached operations of barometric mensuration should, if possible, be performed with a due regard to the prescribed boars of the meteorologist.

The observation of the barometer almost neoessarily implies an inspection of the thermometer, and the height of that instrument should be recorded at the same periods; in addition to which the maximum and minimum, by register thermometers, should be carefully noted. The proper precautions to be taken in placing the instruments for this purpose, are now so well understood that it is needless to repeat them: they are summed up by saying, that they should be sheltered from every species of radiation. The register of the force of radiation in a reflector (as described in the Essay upon Radiation,) and the power of the sun's rays upon black wool would also be particularly intersting; in addition to which, those who have the opportunity should not neglect the variations of the temperature of the sea and other deep bodies of water.

The periods of the barometric observations are also well adapted to those of the hygrometer; but the mean pressure of the aqueous atmosphere should be calculated from the dew-point at 3 P.M., and the lowest temperature at night of the sheltered thermometer. The prognostications to be derived from

[page] 374

this instrument have been already described in the Essay upon the Hygrometer, and to these I shall only now add, that by comparing the dew-point with Table III, of the Essay upon the Climate of London, an accurate estimation may be formed of its accordance with the mean, and of the consequent probability of precipitation, change of wind, &c.

With respect to the rain and evaporation gauges, and the vane, I can add nothing to the full directions given by Mr, Howard; and can only lament with him, that some effectual means have not yet been adopted for measuring the force of the aërial currents as well as their direction. Nor have I, at present, any thing to offer upon the extremely important subject of atmospheric electricity. This interesting department of the science has been almost totally neglected, and it is much to be wished that some competent person would devise the proper means of prosecuting an experimental investigation of the subject.

In concluding these observations, I must not, in justice, omit to state, that in all the practical details with which I have been engaged, I have met with the most ready and able assistance from Mr. Newman. He entered fully into all my views with respect to the improvement of meteorological instruments, and has bestowed much time and attention upon, executing the hints which I have suggested. His portable barometers with iron cisterns may be depended upon for the nicest experiments.

I must terminate these remarks, as I began, by an urgent recommendation to meteorologists to use standard instruments, to observe them with care,

[page] 375

and to make all necessary corrections for accidental differences; and, above all, to keep their tables upon the same scheme. Much curious information is dependant upon such an extensive plan of comparative observation; and without it the observer does little more than accumulate an overwhelming mass of crude and incorrect materials, already too large for arrangement and correction. The example has been set by the Royal Academy of Sciences of Paris, and no better model can be taken than the Meteorological Journal kept at their observatory.

[page 376]

BAROMETRICAL EXPERIMENTS

UPON HEIGHTS

THE late Professor Playfair, in his elaborate Essay upon Barometrical Measurements, has suggested* the idea of fixing two barometers, the one at the top, and the other at the bottom of a high tower or hill of moderate elevation; to be observed at the same instant, together with their corresponding thermometers, for the purpose of computing from the variation of the difference of their heights the quantity of moisture dissolved in the air. "The height at which the one barometer," he observes, "should be placed above the other, ought not to be so small that the unavoidable errors of observation (which may amount to five feet) may be considerable in respect of the whole; nor so great as to introduce error from other causes. It ought not, therefore, to be less than 100, nor much greater than 500 feet." He concludes, "Nor can this application of the barometer fail of leading to some useful conclusion; for if, on trial, it shall be found that the operation of humidity in changing the specific gravity of the air is overruled or concealed by the action of more powerful causes, the discovery, even of this fact, will give a value to the observations."

This suggestion is no longer required for the pur-

* Works of John Playfair, Esq., vol. iii., p. 85.

[page] 377

poses of hygrometry, as we have now the means of accurately appreciating the effects of moisture upon the air: but there is no doubt that it might be applied to the discovery of is other atmospheric influences. For this purpose it is now particularly fitted, as the before unknow hygrometric correction may be independently applied with certainty; and any other disturbances are disengagex from this source of ambiguity.

I have long wished for au opportunity of making the attempt with all the requisite precautions; but, as it is one which requires patient and very careful co-operation, I have not been able to execute the details satisfactorily. The following experiments, however, though necessarily deficient in precision, may not be without interest, and their results may possibly induce others to undertake an investigation which promises amply to repay a patient pursuit.

I have been extremely anxious to ascertain, in the first place, to what degree of precision it is possible to arrive in barometrical mensuration, in different states of the atmosphere, when the corrections for temperature and vapour have both been made; and I availed myself of a long residence in the neighbourhood of Box-hill, and Leith-hill, in Surrey, to decide the point, as far as these heights would permit me; and also to select a series of stations for further experiments. The great disadvantage that I have had to contend with, consists in the want of contemporaneous observations; in lieu of which I have been obliged to substitute the mean of two observations, at the lower station, at setting out and returning. I shall here give the details of four mea-

[page] 378

surements, of Leith-hill, to shew the degree of uncertainty which attaches to this method of proceeding. The heights of the barometer are corrected for all adventitious circumstances, every precaution was taken in observing them, and the instrument is one upon the accuracy of which I can confidently rely.

The lower station was at the foot of Box-hill, about forty-five feet above the bed of the river Mole; and the upper station, the tower upon Leith-hill, about seven miles distant:—

[page] 379

TABLE I. Barometrical Measurements of Leith Hill.

Date.
1822.
Barometer. Temperature. Results. OBSERVATIONS.
Lower Station. Upper Station. Air. Dew Points. Height in Feet. Weight of Col. of Dry Air.
June 24
return
29.924
29.926


29.057
71
74
69
58
60
59


841


.944
Very fine, with some heavy clouds
and heat drops. Not clear.
Jule 1
return
29.993
29.995


29.128
66
66
64
44
47
43

 
823

 
.933
Very fine and clear.
August 12
return
29.748
29.729


28.875
65
68
65
64
62
60


842


.936
Close and damp, with small rain.
Dec. 30
return
29.787
29.750


28.819
29
29
26
15
15
15


838


.940
Very fine and very cold.
836 .938 Mean.

[page] 380

The differences from the mean, exhibited by this Table, must be acknowledged to be very small, being only six feet in 836 feet, or .006 in. of mercury in the weight of the intercepted column of air, corrected for vapour and temperature.

The next series of Experiments was made upon a less elevation, but one which offered the advantage of easier access, and a smaller interval between the observations. The lower station was the same as before, and the upper, a clump of trees upon Hedley Heath, which form a very conspicuous landmark for a large extent of surrounding country. This height I divided into three stations, one above the other, for the purpose of ascertaining whether the parts of a height, so measured in divisions, would correspond with the direct measurement at two observations; in what part the error, if any, was most likely to occur; and also the effect of difference of position with regard to the surrounding hills. The first station above the point of departure was in a deep ravine, below the military road which leads to the top of Box-hill. It is surrounded, except at a narrow entrance, by very steep hlis, those on each side being about 200 feet high. It was selected for the purpose of ascertaining whether a difference in the velocity of the wind passing over such a hollow, would produce any difference in the pressure of the atmospheric column. The second station was on the top of Box-hill, almost perpendicularly above the point from whence I set out; and the third, the trees before described upon the edge of the hill, which is very steep, and forms part of the boundary of a valley which runs at right angles to the one

[page] 381

which is overlooked by die second station, I shall not attempt to give the particulars of the observations, which would occupy too much space, but only the calculated results, and such circumstances as may be supposed to have had an influence in their production. Each height was calculated from the mean of two observations, one made in the ascent and the other on the return. They are included in the following Table:—

[page] 382

Height of Ravine above the first Station. Difference from Mean. Height of Box Hill above the Ravine. Difference from Mean. Height of Hedley above Box Hill. Difference from Mean. Height of Box Hill by direet Observation. Difference from Mean. Height of Box Hill by two Stations. Difference from Mean. Height of Hedley by direet Observation. Difference from Mean. Height of Hedley by three Observation. Difference from Mean. Temp. OBSERVATIONS.
Air. Dew Point.
165 + 7.5 261 – 6.8 159 + 1 2 427 + 2.4 426 + 0.7 586 + 3.9 585 + 1.9 33 29 Wind little and very cold.
168 + 10.5 272 + 4.2 161 + 3.2 441 + 16.4 440 + 16.7 600 + 17.9 601 + 18.9 69 60 Wind South and very high.
148 – 9.5 268 + 0.2 147 – 10.8 416 – 8.6 416 – 9.3 563 – 19.1 563 – 20.1 63 60 Wind S.W. and calm.
150 – 7.5 270 + 2.2 157 – 0.8 421 – 3.6 420 – 5.3 578 – 4.1 577 – 6.1 61 52 Wind W. and little
156 – 1.5 265 – 1.2 155 – 1.2 416 – 8.6 421 – 4.3 571 – 11.1 576 – 7.1 66 52 Ditto ditto
158 – 0.5 271 + 3.2 168 + 10.2 427 + 2.4 429 + 3.7 595 + 12.9 597 + 13.9 81 66 Wind brisk on the Hill, and very hot.
157.5 267.8 157.8 425.3 582.1 583.1 Means

[page] 383

Of these observations, the first series was made at a time when the atmosphere was in such a state, as to require the smallest possible correction for temperature and moisture; and it will be observed, that the calculation from than of the height of the highest station, scarcely differs from the mean, the error being less than two feet in 585. The height of the next lower station also corresponds very closely; but in the height of the first station, we have a difference of 7.5 feet in 157. This difference, most probably, attaches to the observation in the ravine: for, if it had been in the first observation, it would have been discoverable in all the results; whereas, omitting the second, all the rest are correct, and the third is deficient exactly the quantity which is in excess in the second.

The last series forms the proper contrast to this, as requiring almost the greatest possible correction for both temperature and moisture; and here we perceive that the first and second heights are very correct, but there is a large error in the third, amounting to ten feet, in 158. The height of Box-hill thus differs from the mean, only 2.4 feet; while that of Hedley-heath differs thirteen feet; the error must, consequently, be included in the last observation.

In the second series of observations we find an error of eighteen feet in the total elevation; but by attending to the analysis, we cannot, as in the two last cases, trace it to any particular station: it is largest at the first, but goes on accumulating at all: during this series, the wind was extremely violent.

The third set of observations exhibits great errors in deficiency in the first and last sections of the ele-

[page] 384

vation, but none in the intermediate. The results of the fourth are all pretty accurate; and those of the fifth present the only instance of any considerable difference between the measurement at one operation, and the measurement in parts.

The result of all the observations taken together prove, that the intermediate station was very considerably less liable to error than either of the extremes; and strongly suggests the following query:— Whether local currents of air, and those deflections of the wind, which are caused by the different directions of different valleys, may not produce various partial adjustments of density, which may have an influence upon barometrical mensuration?

The measurement of a height, in divisions, does not appear, by this analysis, to be liable to any objections; and it possesses this great advantage, when the altitude is very considerable, viz., that we can make a much nearer approximation thereby, to the real specific gravity of the interceped column of air, than by two observations only. this remark is applicable to the correction for temperature, but much more so that for moistre; for, as we have seen in the previous investigation, like the heat with the elevation, but continues of nearly equal clasticity to a certain height, and then suddenly decreases considerably. the mean, therefore, of and at the top, might be very far, indeed, removed from the real state of the aërial column; and the more we multiply observations of the dew-point, the more we diminish the chances of error from this source.

The last set of experiments includes a series of

[page] 385

forty-five observations upon one height, under every possible variation of the atmosphere. The station was that upon Box-hill, which formed the second stage of the previous set. The results I shall duvide into different classes, to ascertain the influence of different circumstances, and I shall express them by the length of the column of mercury, which would be the equipoise of the intercepted column of air, supposing it corrected for temperature and moisture. The mean result of all the observations is .4848 inch.

The following Table includes the eleven observations, in which both corrections were at the greatest amount.

TABLE III. Barometrical Measurement of Box-hill in very hot Weather.

Length of Column. Temperature. Dryness.
Air. Dew-Point.
.486 81 66 15
.485 68 59 9
.494 68 60 8
.470 67 61 6
.472 67 64 3
.495 65 55 10
.499 65 55 10
.495 65 57 8
.493 65 60 5
.479 65 59 6
.470 65 52 13
.4852 Mean.

2 C

[page] 386

The average scarcely differs from the general mean.

Seven observations in cold weather, in which the required corrections were very small, and mostly on the contrary side, are included in the following Table:—

TABLE IV. Barometrical Measurement of Box-kill in very cold Weather.

Length of Column. Temperature. Dryness.
Air. Dew-Point.
Inch.
.492
31 29 2
.494 34 30 4
.487 30 23 7
.486 29 28 1
.490 32 31 1
.474 27 25 2
.467 32 30 2
.4842 Mean.

The average of these, again, only differs .0006 inch from the same standard. These experiments may, therefore, be regarded as decisive of the adequacy of the corrections for temperature and vapour.

The following eight results were calculated from observations when the moon was nearly upon the meridian:—

[page] 387

TABLE V. Barometrical Measurement of Box-hill, with the Moon with the Meridian.

Length of Column.
488
.483
.469
.474
.479
.486
.480
.470
. 4778 Mean.

Here we have a small but decided difference, sufficient to strengthen the query already suggested in the first Essay. Does not the position of the moon influence, in some degree, the results of barometrical mensurations? The difference, .007 inch, is in deficiency, and agrees, so far, with the anticipation of the effect.

The position of the sun may, also, be expected to have an influence upon the elastic fluids of the atmosphere, independent of its heating power; to determine which, the following observations at noon were extracted:—

2 C 2

[page] 388

TABLE VI. Barometrical Measurement of Box-hill, with the Sun upon the Meridian.

Length of Column.
.473
.470
.495
.475
.465
.476
470
486
.494
474
.47788 Mean.

The difference is the same as in the last Table, and points to the same kind of planetary influence. It is sufficient to justify the query—Does not the position of the sun affect the results of barometrical mensurations?

A third disturbing cause we cannot but look for in the operations of the electric fluid.

The following four observations were made when the atmosphere was highly charged, and just before the commencement of violent thunder-storms:—

[page] 389

TABLE VII Barometrical Measurement of Box-hill during Thunder-storms.

Length of Column.
.481
.485
.476
.465
.4767 Mean.

The experiments, it must be acknowledged, are not sufficient to establish the fact; but the mean difierence, it will be observed, is more than a sixth of the total result, and strongly calls for further inquiry—Whether the electric state of the atmosphere does not affect the results of barometrical mensurations?

The following Table exhibits the barometrical results in the most opposite states of the wind, viz., when very high and when perfectly calm.

TABLE VIII. Barometrical Measurement of Box-hill in different States of the Wind.

Length of Column.
Wind high. Calm.
.492 .475
.499 .465
.495 .476
.470 .470
.494 .472
.4900 .4716 Means.

[page] 390

The differences of +,0052 in wind, and –.0132 in calm weather, induce me to conclude my queries by proposing the following question: What is the effect of wind upon barometrical mensurations? If I had had the means of prosecuting these inquiries in the complete manner which the nicety of the subject requires, I would not have suffered them to retain the form of crude speculations: but, under all circumstances, I am not without hopes that this premature publication may be useful. It may possibly illustrate Mr. Playfair's suggestion; it may indicate the objects which it is calculated to illustrate, and it exemplifies the method of proceeding.

[page 391]

METEOROLOGICAL JOURNAL.

NOTE.—All the maxima results have the sign +, and all the minima - prefixed.

[page] 392

1819 September. Morning. Afternoon. Night.
Barometer. Hygrometer. Weather. Barometer. Hygrometer. Weather. Barometer. Hygrometer. Weather.
1 29.52 62 42 20 fine 29.59 62 42 +20 overcast 29.62 57 42 15 veryfine
2 29.67 63 43 20 overcast 29.67 67 48 19 ditto rain
3 29.59 68 +65 3 showers 29.68 71 61 10 showers 29.75 61 60 1 very fine
◯ 4 29.87 66 59 7 very fine 29.85 70 63 7 overcast 29.83 64 64 rain
5 29374 63 62 1 rain 29.75 65 63 2 fine 29.76 56 54 2 very fine
6 29.87 62 46 16 very fine 29.93 66 46 20 very fine 29.96 55 48 7 same
7 29.97 65 63 2 overcast 29.97 71 64 7 lowering 30.00 66 64 2 overcast
8 30.03 70 62 8 fine 30.04 70 60 10 clearing 30.04 62 59 3 very fine
9 30.05 68 59 9 same 30.02 71 58 13 overcast 30.03 63 61 2 overcast
10 30.01 67 59 8 fine 29.96 70 55 15 very fine 29.97 59 56 3 very fine
11 30.00 63 57 6 overcast 30.02 64 57 7 overcast 30.07 61 56 5 overcast
12 30.14 64 52 12 very fine 30.13 67 47 20 very fine 30.16 52 47 5 very fine
13 30.23 63 49 14 smae 30.20 66 50 16 smae 30.20 56 52 4 smae
14 30.19 62 57 5 fine 30.13 72 55 15 smae 30.8 61 58 3 smae
15 29.92 66 59 7 smae 29.82 68 65 3 showers 29.73 65 64 1 rain
16 29.66 55 50 5 showers 29.70 56 41 15 fine 29.79 48 39 9 very fine
17 29.97 58 47 11 very fine 30.03 59 45 14 very fine 30.09 52 47 5 overcast
18 30.14 60 52 8 overcast 30.10 63 55 8 fine 30.13 53 51 2 very fine
⊕ 19 30.15 60 40 20 fine 30.14 57 -37 20 very fine 30.18 49 43 6 same
20 30.27 56 46 10 same 30.31 52 45 7 same 30.35 50 45 5 same
21 +30.41 55 47 8 very fine 30.41 56 45 11 same 30.40 50 45 5 same
22 30.39 56 48 8 same 30.30 59 46 13 overcast 30.29 54 49 5 fine
23 30.20 56 47 9 overcast 30.07 59 46 13 same 30.01 56 50 6 overcast
24 29.86 58 52 6 same 29.74 61 47 14 very fine 29.59 53 50 3 very fine
25 29.57 58 57 1 showers 29.51 60 58 2 rain -29.50 55 55 rain
26 29.52 59 51 8 fine 29.57 58 46 12 very fine 29.63 53 48 5 dull
27 29.57 61 55 6 same 29.61 63 60 3 showers 29.62 60 60 rain
28 29.58 65 59 6 showers 29.61 60 58 2 rain 29.59 60 60 same
29 29.55 60 60 rain 29.62 60 54 6 fine 29.67 58 57 1 showers
30 29.76 63 60 3 showers 29.79 66 62 4 showers 29.79 63 60 3 overcast
Means 29.913 62 53½ 8.2 29.909 63 52½ 10.2 29.925 56½ 53

[page] 393

1819 September. Temperature. Wind. Rain.
Max. Min. Sun. Rad. Direction. Force.

SEPTEMBER.

Mean Temperature 58° Weight of vapour in inches a cubic foot.

— Pressure 29.915 inches

— Dew-point 52° Mean 4.774 grs.

— Force of vapour 0.428 inches Maximum 7.187 "

— Degree of dryness 6° Minimum 2.861 "

— Degree of moisture 813°

Least observed degree of moisture 503

WINDS.

N.3=43° N.E.6=48° E.1=49° S.E.4=55° S.O.S.W.8=60°
W.2=55° N.W. 6=51°

Amount of rain 2.11 inches

—— of evaporation 2.94 inches

REMARKS.

The sudden fall of the barometer, at the latter part of the month, was accompanied by high winds.

The westerly winds, which constitute half the amount of the month, average more than their mean quantity of vapour; and it was during their prevalence that the rain fell. The other winds are below the mean.

The weather fine, warm, and seasonable.

The depression of the mercnrial column was accompanied by considerable wind, especially during the night.

1 66 46 N W brisk
2 68 60 ditto ditto
3 +74 52 W same 0.07
○ 4 70 60 S W same 0.05
5 66 48 N W same 0.29
6 66 47 ditto same
7 72 61 S W same
8 74 58 N W little
9 72 59 S E same
10 70 52 ditto ditto
11 64 56 N E ditto
12 67 45 same ditto
13 66 49 S E calm
14 72 53 same little
15 68 53 S W ditto 0.06
16 56 43 N brisk 0.19
17 61 50 same little
18 65 46 N W ditto
⊕19 61 41 N ditto
20 60 - 40 N E brisk
21 61 42 same ditto
22 62 50 same ditto
23 61 51 same ditto
24 64 46 E ditto
25 63 50 S W little 0.32
26 61 48 W same 0.13
27 66 57 S W stormy 0.44
28 66 55 same same 0.40
29 64 56 same brisk 0.16
30 66 58 same ditto
Means 65 51 20.11

[page] 394

1819 October. Morning. Afternoon. Night.
Barometer. Hygrometer. Weather. Barometer. Hygrometer. Weather. Barometer. Hygrometer. Weather.
1 29.78 67 59 8 fine 29.73 68 63 5 overcast 29.73 62 60 2 very fine
2 29.67 68 59 9 showers 29.71 65 57 8 fine 29.72 60 58 2 fine
◯ 3 29.69 65 59 6 overcast 29.64 63 56 7 showers 29.60 57 56 1 showers
4 29.53 62 56 6 same 29.56 56 51 5 fine 29.62 46 42 4 very fine
5 29.88 47 41 6 very fine 29.99 48 33 +15 very fine 30.06 46 39 7 same
6 30.09 53 46 7 overcast 29.95 54 52 2 showers 29.94 53 52 1 showers
7 29.89 56 55 1 same 29.88 60 55 5 same 29.91 57 56 1 overcast
8 30.00 63 59 4 same 30.00 64 61 3 overcast 30.00 59 59 rain
9 29.92 60 56 4 same 29.80 61 57 4 showers 29.76 59 57 2 overcast
10 29.78 65 63 2 fine 29.78 70 +66 4 fine 29.79 62 61 1 same
11 29.82 66 60 6 same 29.84 67 61 6 very fine 29.88 58 57 1 very fine
12 29.96 61 60 1 very fine 29.97 70 63 7 same 29.97 60 60 small rain
13 29.93 65 61 4 overcast 29.94 63 59 4 fine 29.99 53 51 2 very fine
14 30.04 56 51 5 dull 30.08 60 56 4 dull 30.11 55 55 showers
15 30.23 55 51 4 very fine 30.26 56 45 11 fine +30.29 49 49 fog
16 30.19 54 50 4 dull 30.17 51 39 12 dull 30.07 46 42 4 very fine
17 30.09 45 39 6 very fine 30.12 46 39 7 very fine 30.15 42 39 3 same
18 30.18 46 41 5 same 30.18 49 39 10 same 30.17 39 39 fog
⊕19 30.09 44 41 3 fog 29.99 49 45 4 fine 29.90 50 49 1 same
20 29.66 56 56 showers 29.56 52 52 rain 29.56 51 51 rain
21 29.50 42 41 1 overcast 29.50 39 37 2 snow 29.38 37 37 sleet
22 29.40 35 33 2 fine 29.41 42 42 rain 29.43 41 41 rain
23 29.33 40 39 1 same 29.30 47 46 1 showers 29.30 41 40 1 very fine
24 -29.30 42 41 1 same 29.32 44 39 5 dull 29.37 39 33 6 dark
25 29.59 41 34 7 same 29.45 42 35 7 fine 29.48 36 35 1 fog
26 29.57 44 42 2 overcast 29.63 42 35 7 very fine 29.71 35 -32 3 very fine
27 29.75 37 33 4 very fine 29.75 43 43 small rain 29.76 38 37 1 dull
28 29.75 39 38 1 fine 29.75 43 41 2 foggy 29.71 34 33 1 fine
29 29.54 36 36 rain 29.51 42 42 rain 29.50 41 89 2 rain
30 29.52 42 42 same 29.53 43 43 same 29.61 46 46 same
31 29.76 48 48 same 29.77 47 47 same 29.82 47 47 same
Means 29.788 51.6 48 3.5 29.776 53 48.3 4.7 29.783 48.8 46.8 1.5

[page] 395

1819 September. Temperature. Wind. Rain.
Max. Min. Sun. Rad. Direction. Force.

OCTOBER.

Mean Temperature 48.6° Weight of vapour in a cubic foot.

— Pressure 29.782 inches

— Dew-point 45.3° Mean 3.869 grs.

— Force of vapour 0.344 inchea Maximum 7.389 "

— Degree of dryness 3.3° Minimum. 2.550 "

— Degree of moisture. 900°

Least observed degree of moisture. 595

WINDS.

N. 6=38° N.E. 4=40° E. 2=45° S.E. 2=60° S. 3=58° S.W. 4=58°
W. 4=51° N.W. 6=42°.

Amount of rain 2.18 inches

— of evaporation 1.178 inches

REMARKS.

The weather, during the first half of the month, mostly very fine and warm; but during the remainder, cold and cloudy.

On the 21st it snowed for two hours in the morning, and two or three inches fell during the night. Snow has not fallen so early in the season for seven or eight years past. The appearance of mid-winter, while the lenves were still upon the trees, was very striking. Much damage was done to fruit and forest trees by the great weight. The Aurora Borealis was seen the month.

1 +68 58 S little
2 68 56 smae ditto
◯ 3 68 51 S W brisk 0.06
4 62 38 N W same
5 50 39 smae ditto 0.04
6 55 52 W squally
7 60 52 W brisk
8 65 53 S W little 0.02
9 61 55 S brisk
10 70 57 S W little
11 68 52 S E same
12 70 57 same same
13 67 47 W calm
14 60 46 N W ditto 0.05
15 59 44 N E little
16 55 38 35 N brisk
17 50 37 33 same same
18 53 32 same same
⊕ 19 53 46 W little
20 57 40 S W same 0.35
21 43 30 28 N W stormy 0.14
22 42 32 25 ditto
23 47 32 27 little
24 44 32 30 N high
25 42 30 22 little
26 44 -27 -17 same
27 43 30 23 N E same 0.06
28 43 31 25
29 43 39 36 high 0.30
30 44 44 44 E same 0.66
31 47 45 48 little 0.50
Means 54.8 42.4 29.8 2.18

[page] 396

1819 November. Morning. Afternoon. Night.
Barometer. Hygrometer. Weather. Barometer. Hygrometer. Weather. Barometer. Hygrometer. Weather.
1 29.79 45 45 rain 29.70 47 46 1 rain 29.66 42 42 overcast
◯ 2 29.65 44 44 fine 29.65 43 34 9 fine 29.65 39 38 1 fine
3 29.82 40 36 4 very fine 29.88 41 35 6 very fine 29.92 36 35 1 same
4 29.88 46 41 5 fine 29.85 51 46 5 overcast 29.80 46 44 2 same
5 29.68 51 44 7 same 29.57 52 51 1 rain 29.50 49 49 fine
6 29.48 47 46 1 same 29.40 48 45 3 fine 29.33 45 41 4 very fine
7 29.39 46 45 1 same 29.45 45 43 2 showers 29.47 38 37 1 fine
8 29.52 40 40 same 29.55 42 36 6 overcast 29.72 35 31 4 very fine
9 29.80 34 31 3 very fine 29.74 40 36 4 very fine 29.60 42 36 6 overcast
10 29.60 47 46 1 overcast 29.32 47 46 1 rain 29.37 44 44 rain
11 29.58 46 45 1 rain 29.70 43 43 same 29.84 43 41 2 showers
12 29.89 42 39 3 showers 29.88 44 39 5 dark 29.82 42 41 1 dark
13 29.78 44 38 6 overcast 29.73 45 44 1 overcast 29.73 44 44 rain
14 29.75 43 43 same 29.75 44 42 2 same 29.75 44 42 2 dark
15 29.72 44 44 fog 29.65 46 46 fog 29.57 47 47 rain
16 29.50 41 41 rain 29.41 40 40 rain 29.40 36 36 same
⊕ 17 29.64 42 42 small rain 29.73 43 43 small rain 29.91 43 43 dark
18 29.99 42 34 8 very fine +30.01 41 32 +9 very fine 30.00 33 31 2 very fine
19 29.91 34 33 1 overcast 29.83 36 31 5 overcast 29.80 36 31 5 overcast
20 29.66 35 26 9 fine 29.40 36 35 1 same 29.12 42 42 sleet
21 -29.08 40 39 1 same 29.13 41 41 showers 29.33 36 34 2 clearing
22 29.46 32 30 2 very fine 29.49 34 32 2 fine 29.54 30 30 very fine
23 29.68 29 27 2 same 29.72 33 30 3 very fine 29.74 29 29 same
24 29.81 27 -25 2 same 29.89 36 33 3 same 29.94 32 32 same
25 29.92 31 31 same 29.86 34 33 1 same 29.71 31 31 fog
26 29.64 36 35 1 rain 29.67 37 37 rain 29.76 33 33 overcast
27 29.83 36 36 overcast 29.83 37 36 1 misty 29.83 29 29 fog
28 29.79 31 31 fog 29.72 33 33 fog 29.66 41 41 rain
29 29.61 49 49 rain 29.60 50 50 rain 29.66 50 50 same
30 29.64 51 +51 same 29.58 51 51 same 29.58 47 47 overcast
Means 29.683 40.5 38.5 1.9 29.656 42 39.6 2.3 29.655 39.4 38.3 1.1

[page] 397

1819 November. Temperature. Wind. Rain.
Max. Min. Sun. Rad. Direction. Force.

NOVEMBER.

Mean Temperature 39.2° Weight of vapour in in a cubic foot.

— Pressure 29.664 inches

— Dew-point 37.4° Mean 2.967 grs.

— Force of vapour 0.259 inches Maximum. 4.684 "

— Degree of dryness 1.8° Minimum. 2.019 "

— Degree of moisture. 941

Least degree of observed moisture 739

WINDS.

N. 3=36° N E. 6=38° E. 2=43° S.E. 0 S.0 S.W. 5 =45° W. 4=41° N.W. 10=34°

Amount of rain 2.150 inches

— of evaporation 0.480 inches

REMARKS.

The weather, for the most part, uncommonly cold and cloudy, with frequent fogs, and sharp hoar frosts.

The sudden increase of temperature on the 29th, with a south-west wind, is very remarkable.

1 47 38 32 E little 0.02
◯ 2 44 33 26 N W ditto
3 41 33 26 —— ditto
4 51 34 30 W ditto
5 +52 43 40 S W ditto 0.17
6 49 38 34 —— brisk
7 46 34 27 W little
8 43 28 21 N same
9 41 38 35 W brisk
10 48 43 40 N W little 0.15
11 46 37 32 N E high 0.24
12 45 41 38 —— brisk 0.05
13 45 41 38 —— same
14 45 41 40 —— little
15 46 41 38 W ditto 0.10
16 41 36 38 N little 0.49
⊕ 17 43 40 38 E brisk 0.09
18 42 31 27 N E high
19 37 33 30 —— little
20 42 37 29 N W brisk
21 41 28 22 —— same
22 35 26 18 —— little
23 34 -23 -15 —— ditto
24 36 28 20 —— ditto
25 36 28 21 —— ditto
26 37 32 27 N dittlo
27 37 29 27 N W brisk
28 41 33 31 S W little 0.57
29 51 48 45 —— brisk 0.15
30 51 46 43 —— ditto 0.12
Means 43.1 35.3 30.9 2.15

[page] 398

1819 December. Morning. Afternoon. Night.
Barometer. Hygrometer. Weather. Barometer. Hygrometer. Weather. Barometer. Hygrometer. Weather.
◯ 1 29.72 48 47 1 overcast 29.91 46 43 3 fine 29.97 37 37 fine
2 29.91 48 48 same 29.89 47 47 rain 30.00 38 38 same
3 30.08 35 33 2 very fine 30.07 39 39 fog +30.09 39 39 very fine
4 29.63 49 46 3 fine 29.50 44 44 rain 29.52 42 42 rain
5 29.73 41 38 3 dark 29.74 40 39 1 dark 29.89 35 35 dark
6 29.97 37 37 same 29.97 36 36 same 29.98 35 33 2 same
7 29.95 33 33 sleet 29.90 33 33 small rain 29.90 34 34 same
8 29.94 29 22 7 clearing 29.96 28 18 +10 very fine 29.97 21 18 3 very fine
9 29.98 24 23 1 fine 29.96 29 24 5 overcast 29.90 27 27 snow
10 29.83 30 29 1 overcast 29.88 30 29 1 same 29.93 21 21 fine
11 29.92 18 -15 3 very fine 29.93 23 21 2 fine 29.94 23 21 2 very fine
12 29.89 32 30 2 same 29.83 33 33 sleet 29.78 31 31 snow
13 29.72 26 25 1 very fine 29.70 31 31 very fine 29.70 27 27 fine
14 29.66 25 25 fine 29.54 33 33 rain 29.49 30 30 fine
15 29.45 32 32 same 29.42 35 34 1 same 29.55 33 33 very fine
16 29.79 31 31 same 29.89 34 34 misty 29.91 33 33 rain
⊕ 17 29.66 37 37 rain 29.38 44 44 rain 29.37 50 50 same
18 29.35 51 51 dull 29.48 52 52 same 29.62 50 49 1 clearing
19 29.77 51 51 rain 29.76 53 53 rain 29 69 53 53 rain
20 29.62 53 +53 same 29.60 52 52 same 29.66 52 52 rain
21 29.86 46 41 5 fine 29.78 42 42 same 29.66 51 51 same
22 29.64 51 51 rain -29:66 49 48 1 fine 29.46 50 50 same
23 29.26 42 42 same 29.26 40 35 5 same 29.29 36 36 fine
24 29.30 32 32 fine 29.26 33 33 fog 29.26 31 31 very fine
25 29.31 32 31 1 same 29.33 31 27 4 fine 29.39 29 25 4 same
26 29.42 27 25 2 same 29.43 29 27 2 very fine 29.43 30 30 fine
27 29.40 31 31 same 29.40 31 29 2 fine 29.42 30 29 1 same
28 29.43 34 34 dull 29.43 32 32 snow 29.46 31 30 1 overcast
29 29.55 27 27 misty 29.57 29 29 haze 29.54 23 23 fine
30 29.48 25 25 fog 29.43 31 31 fog 29.37 32 32 overcast
◯ 31 29.32 27 24 3 fine 29.31 31 26 4 very fine 29.30 21 21 very fine
Means 29.662 35.6 34.4 1.1 29.650 36.7 35.4 1.3 29.659 34.6 34.2 0.4

[page] 399

1819 December. Temperature. Wind. Rain.
Max. Min. Sun. Rad. Direction. Force.

DECEMBER.

Mean Temperature 34.3° Weight of vapour in a cubic foot.

— Pressure 29.657 inches

— Dew-point32.9° Mean 2.624 grs.

— Force of vapour 0.217 inchea Maximum. 5.003 "

— Degree of dryness 1.4° Minimum. 1.552 "

— Degree of moisture. 923°

Least observed degree of moisture 696

WINDS.

N. 2=25° N.E. 3=32° E. 3=29° S.E. 2=30° S. 1=43° S.W. 12 =40° W. 4=31° N.W. 4=31°.

Amount of rain 1.18 inches

— of evaporation 0.558 inches

REMARKS.

The first four days were very mild; but with the change of wind, on the fifth, a sharp winter may be said to have commenced. The frost lasted till the 17th, when a thaw took place, and mild (and to the fellings oppressive) weather continued to the 23d. The frost set in again on the 25th, and continued very sharp, with fine weatherm to the end of the month.

The Aurora Borealis was observed in the neighbourhood of London on the 14th.

◯ 1 48 36 28 N W little
2 48 30 25 S W brisk
3 39 34 29 —— little
4 49 39 39 —— brisk 0.13
5 41 35 33 E high 0.11
6 37 33 31 N E ditto 0.07
7 34 29 29 —— ditto 0.10
8 29 20 13 E ditto
9 29 27 26 N ditto 0.21
10 32 -17 -12 —— little 0.20
11 24 20 13 N W ditto
12 34 21 15 W ditto 0.17
13 32 22 15 —— ditto
14 33 28 22 S W ditto
15 35 29 25 W brisk 0.03
16 35 30 21 —— ditto
⊕ 17 50 43 40 S ditto
18 +53 48 44 S W ditto
19 53 44 42 —— little 0.09
20 53 44 42 —— high 0.07
21 51 42 41 —— brisk
22 51 42 42 —— ditto
23 42 30 24 N W ditto
24 33 26 19 S E ditto
25 32 24 16 N E ditto
26 30 29 23 N W ditto
27 32 28 25 S E ditto
28 34 25 15 E ditto
29 29 21 14 S W ditto
30 32 25 19 —— ditto
◯ 31 30 17 15 —— little
Means 38.2 30.4 25.9 1.18

[page] 400

1820 January. Morning. Afternoon. Night.
Barometer. Hygrometer. Weather. Barometer. Hygrometer. Weather. Barometer. Hygrometer. Weather.
1 29.38 20 19 1 very fine 29.44 27 27 fog 29.53 25 25 fog
2 29.54 30 30 mist 29.50 35 35 rain 29.38 37 37 rain
3 29.58 30 25 5 fine 29.76 29 25 4 very fine 29.92 26 26 very fine
4 29.98 24 24 fog 29.97 26 24 2 fog 29.98 21 21 misty
5 30.05 20 19 1 mist 30.06 25 25 same 30.05 25 25 fine
6 30.03 27 27 fog 30.04 34 34 same 30.06 35 35 rain
7 30.18 32 23 9 dull 30.28 28 21 7 dull 30.35 26 20 6 dark
8 30.42 27 22 5 very fine 30.44 26 21 5 fine 30.52 23 19 4 same
9 +30.59 26 25 1 little snow 30.51 26 23 3 very fine 30.32 28 28 snow
10 30.31 25 24 1 fine 30.24 26 26 snow 30.11 26 26 same
11 29.90 28 28 sleet 29.70 32 32 same 29.67 33 33 dark
12 29.94 25 17 8 clearing 30.03 25 18 7 light snow. 30.10 20 17 3 dull
13 30.05 21 17 4 fine 30.05 28 27 1 mist 30.10 22 17 5 overcast
14 30.12 25 17 8 light snow. 30.09 26 17 9 dull 30.01 21 -10 +11 very fine
⊕ 15 29.79 12 10 2 fine 29.71 21 16 5 fine 29.66 28 26 2 dark
16 29.65 22 22 same 29.68 25 25 mist 29.68 26 25 1 dull
17 29.60 32 32 fog 29.57 32 32 dull 29.51 32 26 6 snow
18 29.39 29 29 snow 29.32 32 32 rain 29.02 35 35 rain
19 -28.89 45 43 2 mild&damp 28.94 43 41 2 dull 28.98 33 33 dull
20 29.50 30 26 4 very fine 29.45 32 24 8 fine 29.13 32 32 snow
21 29.12 35 34 1 sleet 29.26 33 33 showers 29.68 28 25 3 very fine
22 29.96 25 22 3 very fine 30.04 28 26 2 very fine 30.07 22 22 fog
23 30.02 33 33 fog 29.98 36 36 fog 29.90 36 36 overcast
24 29.79 39 34 5 fine 29.71 41 41 showers 29.77 43 43 rain
25 29.79 42 42 showers 29.63 42 42 rain 29.44 42 42 same
26 29.47 44 44 rain 29.59 47 +46 1 same 29.59 46 46 same
27 29.56 47 45 2 very fine 29.50 49 46 3 very fine 29.34 46 46 same
28 29.56 43 42 1 dull 29.73 43 37 6 dull 29.94 41 40 1 showers
29 30.09 40 36 4 fine 30.06 42 36 6 fine 30.05 40 37 3 fine
◯ 30 29.99 42 42 rain 29.99 45 45 rain 29.96 44 44 rain
31 29.92 42 42 very fine 29.91 46 43 3 very fine 29.97 43 43 overcast
Means 29.812 31 28.8 2.1 29.812 83.2 30.8 2.4 29.798 31.7 30.3 1.4

[page] 401

1820 January. Temperature. Wind. Rain.
Max. Min. Sun. Rad. Direction. Force.

JANUARY.

Mean Temperature 30.8° Weight of vapour in a cubic foot.

— Pressure. 29.807 inches

— Dew-point. 29.2° Mean. 2.285 grs.

— Force of vapour. 0.195 inches Maximum. 4.016 "

— Degree of dryness. 1.6° Minimum. 1.179 "

— Degree of moisture. 946°

Least observed degree of moisture 671

WINDS.

N. 2=27° N.E. 7=29° E. 3=17° S.E. 1=27° S. 2=37° S.W. 9=39° W 3=28° N.W. 5=26°.

Amount of rain, &c. 1.83 inches

— of evaporation. 0.341 inches

REMARKS.

The frost continued unusually severe from the first to the twentythird.

The weather was generally cloudy and foggy, and there were frequent snow showers.

On the 23d it broke up, and the remaining eight days were mild and cloudy, with light showers of rain.

The feathered tribes suffered very much from cold and hunger, during the frosts. The Thames was much swollen, and was full of floating ice, of great thickness.

The Aurora Borealis was seen during the month.

1 27 20 12 N W calm
2 37 30 29 S W ditto
3 30 22 16 N W little
4 26 17 14 —— ditto
5 26 22 16 W ditto
6 34 31 31 —— ditto
7 32 24 21 E high
8 27 21 18 N E ditto
9 28 21 19 —— ditto
10 27 25 25 —— brisk
11 33 25 25 S W little
12 25 -11 -5 E brisk
13 28 22 16 N E ditto
14 26 19 14 E ditto
⊕ 15 28 19 13 N E ditto
16 26 22 12 N W little
17 33 27 25 W ditto
18 35 32 30 N W ditto 1.15
19 45 27 21 S W brisk
20 32 32 31 S E ditto
21 35 22 14 N ditto
22 28 19 13 —— ditto
23 36 33 29 S W ditto
24 43 39 36 —— ditto 0.38
25 43 39 35 S ditto 0.13
26 48 44 42 S W ditto 0.07
27 +50 44 40 —— ditto
28 44 37 33 N E variable
29 43 38 37 N W little
◯ 30 45 39 34 S W ditto
31 36 34 28 —— ditto 0.10
Means 34 27.6 23.6 1.83

[page] 402

1820 February. Morning. Afternoon. Night.
Barometer. Hygrometer. Weather. Barometer. Hygrometer. Weather. Barometer. Hygrometer. Weather.
1 29.89 37 37 fine 29.81 40 34 6 very fine 29.77 34 34 very fine
2 29.74 32 32 fog 29.77 35 32 3 dull 29.81 34 34 dull
3 29.87 33 31 2 fine 29.90 34 30 4 same 29.94 33 31 2 same
4 29.95 34 31 3 dull 29.95 37 30 7 same 29.96 33 33 same
5 29.89 40 39 1 overcast 29.85 42 42 rain 29.84 41 41 same
6 29.77 44 44 rain 29.90 47 42 5 fine 29.99 45 45 overcast
7 30.06 47 47 dull 30.06 49 +49 overcast 30.06 46 46 same
8 30.06 45 45 mist 30.04 46 45 1 same 30.06 45 45 same
9 29.95 43 42 1 fine 29.85 46 44 2 very fine 29.79 41 41 rain
10 29.80 46 46 rain 29.90 47 39 8 same 29.99 36 36 very fine
11 30.02 40 38 2 very fine 29.94 45 44 1 fine 29.80 43 43 dull
12 29.80 42 42 rain 29.88 41 41 rain 29.95 40 40 same
13 29.95 42 36 6 fine 29.92 42 34 8 fine 29.99 36 36 same
⊕ 14 30.12 38 34 4 same 30.13 42 31 +11 same 30.16 36 35 1 same
15 30.20 32 32 dull +30.20 36 32 4 fine 30.20 29 29 very fine
16 30.16 28 27 1 very fine 30.10 32 31 1 fog 30.06 28 28 same
17 30.00 26 26 mist 29.99 32 23 9 very fine 29.99 26 -22 4 same
18 30.00 28 26 2 fine 29.99 33 30 3 fine 30.00 31 29 2 dull
19 30.02 34 30 4 same 29.99 33 31 2 same 29.94 32 32 little snow
20 29.80 32 32 snow 29.79 32 32 snow 29.83 32 32 dull
21 29.78 32 31 1 mist 29.76 34 34 sleet 29.69 35 35 rain
22 29.75 35 35 rain 29.69 42 42 fog 29.64 41 41 dull
23 29.56 47 47 same 29.49 46 46 rain 29.46 42 42 overcast
24 29.39 39 39 rain -29.30 38 38 same 29.30 36 35 1 same
25 29.35 36 36 same 29.49 39 39 same 29.70 35 35 rain
26 29.86 35 35 same 29.93 33 30 3 same 30.00 30 28 2 fine
27 30.01 35 26 9 fine 29.97 35 27 8 fine 29.99 29 29 very fine
28 29.90 35 30 5 very fine 29.83 36 30 6 very fine 29.83 29 26 3 same
◯ 29 29.77 29 29 mist 29.68 38 33 5 same 29.51 36 35 1 rain
Means 29.874 36.7 35.3 1.4 29.865 39 35.6 3.3 29.870 35.6 35.1 0.5

[page] 403

1820 February. Temperature. Wind. Rain.
Max. Min. Sun. Rad. Direction. Force.

FEBRUARY.

Mean Temperature 35.9° Weight of vapour in a cubic foot.

— Pressure. 29.869 inches

— Dew-point. 34° Mean 2.699 grs.

— Force of vapour 0.232 inchea Maximum. 4.407 "

— Degree of dryness 1.9° Minimum 1.764 "

— Degree of moisture. 939°

Least observed degree of moisture. 684

WINDS.

N. 0= N.E 10=29° E. 1=32° S.E. 4=36° S. 4=36° S. 4=39° S.W 5=38° W. 5=39° N.W. 0.

Amount of rain, &c. 1.18 inches

— of evaporation 0.43 inches

REMARKS.

The weather, though chiefly cloudy, was, for the most part, fair, with hard frost at intervals.

The winter may be considered as having ended with the deep mow on the 20th.

1 41 27 23 S little
2 35 32 30 E ditto
3 37 32 30 N E ditto
4 37 32 32 S W ditto
5 43 34 34 S ditto 0.02
6 47 40 37 W variable 0.04
7 +49 44 41 S W little
8 46 38 31 —— ditto
9 46 40 35 S ditto
10 47 32 26 W ditto 0.03
11 45 39 36 —— brisk
12 42 38 36 S ditto 0.46
13 42 32 29 S E ditto
⊕ 14 42 31 29 W ditto
15 36 26 23 N E ditto
16 32 23 17 —— ditto
17 32 -21 -16 —— little
18 33 30 27 —— ditto
19 34 22 18 —— brisk
20 32 29 22 S E ditto
21 35 33 33 —— ditto
22 42 39 38 W ditto 0.42
23 47 37 36 S E ditto
24 39 35 34 S W ditto 0.10
25 39 34 33 N E high 0.07
26 35 30 28 —— ditto 0.92
27 35 28 24 —— ditto
28 36 28 20 —— brisk
◯ 29 38 34 31 S W ditto 0.02
Means 39.3 32.4 29.2 1.18

[page] 404

1820 March. Morning. Afternoon. Night.
Barometer. Hygrometer. Weather. Barometer. Hygrometer. Weather. Barometer. Hygrometer. Weather.
1 29.49 43 33 10 very fine 29.51 42 30 12 very fine 29.41 38 31 7 dull
2 29.00 35 28 7 snow 29.13 35 20 15 same 29.46 29 26 3 very fine
3 29.68 33 27 6 very fine 29.76 34 27 7 same 29.90 28 25 3 same
4 29.91 35 27 8 same 29.80 34 33 1 same 29.96 29 29 fine
5 30.12 33 24 9 same 30.13 32 -19 18 same 30.13 25 21 4 very fine
6 30.08 33 27 6 dull 30.06 33 21 12 fine 30.10 26 24 2 fine
7 30.09 35 21 14 fine 30.03 35 35 sleet 30.09 33 32 1 sleet
8 30.16 36 32 4 same 30.14 38 28 10 very fine 30.16 30 30 fine
9 30.09 36 32 4 very fine 30.01 42 34 8 same 29.93 31 31 very fine
10 29.79 36 32 4 fine 29.63 41 31 10 very fine 29.59 31 31 same
11 29.57 38 36 2 very fine 29.51 48 35 13 same 29.49 35 35 same
12 29.40 38 37 1 fine 29.36 42 36 6 same 29.36 38 38 dull
13 29.56 45 39 6 very fine 29.68 48 34 14 same 29.85 37 37 very fine
⊕ 14 30.01 48 45 3 fine 30.07 53 51 2 overcast 30.13 49 49 overcast
15 30.19 58 51 7 same 30.18 59 51 2 very fine 30.22 51 51 same
16 +30.29 48 46 2 dull 30.24 49 41 8 same 30.23 39 38 1 very fine
17 30.16 44 42 2 showers 30.16 50 46 4 fine 30.23 41 41 sleet
18 30.26 42 36 6 very fine 30.21 45 26 19 very fine 30.20 37 35 2 dull
19 30.14 42 37 5 dull 30.11 44 33 11 fine 30.16 37 29 8 same
20 30.16 39 35 4 fine 30.13 46 37 9 same 30.13 35 35 very fine
21 30.06 45 35 10 same 29.93 48 37 11 same 29.85 41 39 2 showers
22 29.74 47 35 12 very fine 29.71 50 33 17 same 29.53 46 46 rain
23 29.30 49 42 7 fine 29.09 50 36 14 same 29.04 41 36 5 very fine
24 29.02 48 36 12 same -28.87 48 37 11 light show. 28.87 38 38 overcast
25 29.21 42 30 12 very fine 29.33 46 23 +23 very fine 29.56 33 32 1 very fine
26 29.67 45 30 15 fine 29.60 45 45 light rain 29.64 48 48 rain
27 29.74 53 49 4 light show. 29.72 54 50 4 showers 29.76 50 50 same
28 29.92 55 49 6 fine 29.94 56 48 8 very fine 29.99 50 50 overcast
◯ 29 30.03 56 49 7 same 29.99 59 52 7 same 29.92 49 49 very fine
30 29.88 53 48 5 same 29.88 59 +54 5 fine 29.91 44 39 5 same
31 29.91 50 42 8 same 29.88 58 37 21 very fine 29.92 47 44 3 same
Means 29.826 43.2 36.5 6.7 29.799 45.9 36.1 9.7 29.829 38.2 36.7 1.5

[page] 405

1820 March. Temperature. Wind. Rain.
Max. Min. Sun. Rad. Direction. Force.

MARCH.

Mean Temperature 40.9° Weight of vapour in a cubic foot.

— Pressure 29.818 inches

— Dew-point. 35.4° Mean 2.790 grs.

— Force of vapour 0.243 inchea Maximum. 5.122 "

— Degree of dryness 5.5° Minimum. 1.587 "

— Degree of moisture. 835°

Least observed degree of moisture 449

WINDS.

N. 1= 28° N.E 9=30° E. 0= S.E.4=33° S. 0= S.W 6=45° W. 3=40° N.W. 8=35°.

Amount of rain, &c. 0.14 inches

— of evaporation 1.48 inches

REMARKS.

The weather throughout the principal part of the month very fine and dry.

On the 2d from five to eight A. M. the wind blew a complete hurricane.

There were many sharp frosts during the first fortnight.

1 43 32 28 N W high
2 35 28 28 —— ditto 0.04
3 34 27 25 N E ditto
4 36 26 22 —— brisk
5 33 25 18 —— ditto
6 33 -24 -18 —— ditto
7 36 33 32 N W little 0.03
8 39 27 21 N E ditto
9 43 29 20 S W ditto
10 42 30 26 S E ditto
11 50 32 29 S E brisk
12 44 35 32 —— ditto
13 50 34 27 W ditto
⊕ 14 56 48 43 S W ditto
15 61 44 43 —— ditto
16 52 36 32 S E ditto
17 51 34 30 N E ditto 0.01
18 47 35 30 —— ditto
19 45 35 33 —— high
20 47 35 28 —— little
21 49 39 35 N W ditto 0.01
22 50 44 42 —— brisk 0.02
23 50 38 36 —— ditto
24 48 33 29 W ditto
25 46 28 22 N ditto
26 48 44 44 S W ditto 0.02
27 54 44 41 —— little 0.01
28 57 48 44 W ditto
◯ 29 +60 41 35 S W ditto
30 60 33 27 N W ditto
31 60 37 31 —— ditto
Means 47 84.7 30.6 0.14

[page] 406

1820 April. Morning. Afternoon. Night.
Barometer. Hygrometer. Weather. Barometer. Hygrometer. Weather. Barometer. Hygrometer. Weather.
1 30.06 51 42 9 overcast 30.02 56 48 8 overcast 30.07 50 50 dull
2 30.10 55 52 3 dull 30.10 63 53 10 dull 30.12 51 50 1 same
3 30.21 60 +53 7 same 30.21 60 53 7 same 30.20 47 47 fine
4 30.04 52 40 12 fine 29.96 60 51 9 fine 29.94 45 38 7 very fine
5 29.85 53 41 12 same 29.78 64 43 21 very fine 29.77 50 42 8 fine
6 29.49 47 45 2 rain 29.46 49 46 3 showers 29.46 36 34 2 very fine
7 29.52 43 32 11 fine 29.54 46 41 5 fine 29.55 37 37 same
8 29.44 48 37 11 same 29.43 49 41 8 showers -29.20 44 44 rain
9 29.37 42 40 2 showers 29.44 51 36 15 light show. 29.54 38 38 very fine
10 29.54 49 42 7 dull 29.49 49 49 rain 29.49 48 48 dull
11 29.62 50 49 1 same 29.61 55 51 4 showers 29.62 48 48 rain
⊕ 12 29.89 50 49 1 dull 29.87 55 49 6 rain 30.2 50 48 2 dull
13 30.01 49 47 2 same 30.00 50 49 1 same 29.84 48 48 rain
14 29.79 50 49 1 same 29.65 50 49 1 same 29.73 46 46 same
15 29.81 55 41 14 very fine 29.87 56 41 15 very fine 30.09 43 39 4 very fine
16 30.40 58 40 18 same 30.22 61 49 11 same 30.33 51 50 1 same
17 30.36 64 53 11 same 30.33 65 52 13 same 30.32 53 51 2 same
18 30.22 58 52 6 same 30.15 62 51 11 same 30.16 48 48 same
19 30.20 62 51 11 same 30.17 67 46 21 same 30.13 54 46 8 same
20 30.19 61 46 15 same 30.23 62 43 19 same 30.28 48 42 6 same
21 30.30 59 44 15 same 30.29 62 44 18 same 30.33 49 49 same
22 30.50 59 44 15 same 30.38 60 44 16 same 30.50 49 43 4 same
23 30.45 58 45 13 same 30.45 60 44 16 same 30.50 46 41 5 same
24 30.49 55 45 10 same 30.51 62 39 23 same +30.54 45 40 5 same
25 30.38 52 42 10 same 30.39 60 34 +26 same 30.32 44 42 2 same
26 30.10 64 44 20 same 29.80 56 44 12 overcast 29.66 47 43 4 overcast
27 29.71 43 43 showers 29.83 42 33 9 same 29.91 39 35 4 dull
◯ 28 30.01 50 -27 23 fine 30.01 51 33 18 very fine 30.03 40 33 7 very fine
29 30.10 55 37 18 very fine 30.10 58 40 18 overcast 30.11 48 44 4 overcast
30 30.18 56 47 9 showers 30.22 56 33 23 very fine 30.27 44 33 11 very fine
Means 30.011 53.6 43.9 9.6 29.983 56.5 44.3 12.2 30.001 46.1 43.2 2.9

[page] 407

1820 April. Temperature. Wind. Rain.
Max. Min. Sun. Rad. Direction. Force.

APRIL.

Mean Temperature 49.6° Weight of vapour in a cubic foot.

— Pressure 29.998 inches

— Dew-point. 42.2° Mean 3.451 grs.

— Force of vapour 0.306 inches Maximum. 4.934 "

— Degree of dryness 7.4° Minimum. 2.063 "

— Degree of moisture. 774°

Least observed degree of moisture 414

WINDS.

N. 1= 31° N.E 3=39° E. 3=44° S.E. 4=47° S. 2=43° S.W 4=44° W. 5=43° N.W. 8=45°.

Amount of rain, &c. 1.65 inches

— of evaporation 2.67 inches

REMARKS.

The first part of the month was remarkably warm and fine, but the weather afterwards became cold, cloudy, and wet. In the middle it again changed, and was fine and seasonable.

The swallows were first seen about the latter end of the month.

1 59 44 41 W high
2 64 49 45 N W little
3 63 36 32 variable
4 60 37 34 S E brisk
5 66 44 41 W ditto
6 52 30 27 S W ditto 0.17
7 50 30 25 ditto 0.02
8 52 41 41 S ditto 0.12
9 52 -29 -23 N W ditto
10 49 46 46 S ditto 0.26
11 59 46 40 S W ditto
⊕ 12 58 46 43 ditto 0.35
13 50 46 45 S E ditto 0.45
14 50 39 34 ditto 0.10
15 57 40 35 N W little
16 63 40 39 W ditto
17 +67 44 39 N W ditto
18 66 43 37 S E ditto
19 67 45 41 N W ditto
20 64 41 35 ditto
21 67 41 36 E variable
22 63 39 36 little
23 62 40 36 ditto
24 64 38 33 N E brisk
25 64 37 29 ditto
26 67 41 38 W ditto
27 47 38 36 N E ditto
◯ 28 53 36 28 N little
29 61 41 34 W ditto
30 59 36 28 N W ditto 0.20
Means 59.1 40.2 35.9 1.67

[page] 408

1820 May. Morning. Afternoon. Night.
Barometer. Hygrometer. Weather. Barometer. Hygrometer. Weather. Barometer. Hygrometer. Weather.
1 30.31 56 41 15 very fine +30.33 56 36 20 very fine 30.33 46 41 5 very fine
2 30.27 58 42 16 fine 30.16 57 41 16 fine 30.14 48 41 7 dull
3 30.15 45 42 3 dull 30.10 50 43 7 dull 30.03 45 40 5 same
4 29.95 49 33 16 very fine 29.88 53 35 18 fine 29.86 42 37 5 same
5 29.88 51 30 21 fine 29.88 52 -30 22 same 29.88 40 37 3 very fine
6 29.88 59 37 22 very fine 29.79 60 39 21 overcast 29.67 46 43 3 showers
7 29.62 62 43 19 same 29.68 65 41 +24 very fine 29.70 53 43 10 dull
8 29.71 62 53 9 fine 29.69 62 53 9 showers 29.64 54 54 showers
9 29.65 64 51 13 same 29.67 63 45 18 very fine 29.67 51 49 2 very fine
10 29.80 68 48 20 very fine 29.88 63 52 11 light show. 29.90 53 52 1 fine
11 29.97 65 50 15 fine 29.96 65 45 20 very fine 30.00 51 46 5 very fine
⊕ 12 30.05 65 46 16 same 30.02 67 47 20 same 30.01 53 53 fine
13 29.95 60 53 7 dull 29.90 60 55 5 showers 29.88 53 53 same
14 29.88 65 53 12 fine 29.82 65 48 17 overcast 29.82 52 49 3 same
15 29.83 67 47 20 very fine 29.80 66 44 22 very fine 29.74 54 54 rain
16 29.70 58 53 5 showers 29.72 61 47 14 showers 29.72 52 52 same
17 29.76 65 49 16 fine 29.73 64 45 19 dull 29.61 53 51 2 very fine
18 29.32 52 51 rain -29.16 56 52 4 showers 29.28 50 50 rain
19 29.72 69 47 22 very fine 29.81 64 46 18 very fine 29.97 51 46 5 very fine
20 30.15 68 56 12 fine 30.25 66 54 12 fine 30.30 53 48 5 same
21 30.32 62 50 12 same 30.30 70 53 17 same 30.29 52 49 3 fine
22 30.15 68 53 15 very fine 30.18 77 56 21 very fine 30.16 59 53 6 very fine
23 30.04 69 57 12 same 29.97 72 57 15 same 29.92 59 55 4 same
24 29.81 69 +58 11 same 29.86 70 54 16 same 29.84 58 50 8 dull
25 29.82 58 53 5 fine 29.80 57 54 3 showers 29.80 50 48 2 same
26 29.86 60 47 13 overcast 29.83 59 52 7 same 29.79 56 55 1 showers
◯ 27 29.71 55 52 3 showers 29.56 59 55 4 same 29.68 52 47 5 showers
28 29.65 59 48 11 overcast 29.62 56 51 5 rain 29.50 52 51 1 rain
29 29.41 57 43 14 same 29.41 53 50 3 hail&thund. 29.40 46 42 4 fine
30 29.40 55 42 13 same 29.41 58 43 15 light show. 29.42 51 43 8 dull
31 29.45 56 42 14 hail 29.46 60 48 12 showers 29.51 45 42 3 very fine
Means 29.844 60.5 47.4 13 29.826 61.5 47.7 14 29.821 50.9 47.5 3.4

[page] 408

1820 May. Temperature. Wind. Rain.
Max. Min. Sun. Rad. Direction. Force.

MAY.

Mean Temperature 55.3° Weight of vapour in a cubic foot.

— Pressure. 29.830 inches

— Dew-point. 46.8° Mean 4.084 grs.

— Force of vapour 0.361 inch Maximum. 5.743 "

— Degree of dryness 8.5° Minimum. 2.257 "

— Degree of moisture 752°

Least observed degree of moisture 444

WINDS.

N. 0= N.E 1=32° E. 2=38° S.E. 4=51° S. 1=54° S.W 15=49° W. 6=46° N.W. 2=40°.

Amount of rain, &c. 2.68 inches

— of evaporation 2.68 inches

REMARKS.

During the first part of the month the weather was mostly fine, warm, and dry; but during the latter part cloudy and dull, with frequent falls of rain, accompanied, at intervals, with gales of wind.

1 62 40 33 N W brisk
2 65 42 39 S E ditto
3 51 41 40 E ditto
4 54 33 28 —— ditto
5 54 -33 -24 N E ditto
6 64 45 44 S W ditto 0.48
7 66 51 46 N W ditto
8 67 51 47 S W little 0.35
9 69 46 42 —— brisk
10 69 50 47 —— ditto
11 69 47 41 W ditto 0.59
⊕ 12 70 43 36 S W ditto
13 64 51 49 S E ditto
14 70 46 41 S W ditto
15 70 49 49 —— little
16 66 47 46 —— ditto
17 67 49 46 —— brisk
18 60 47 47 —— high
19 70 47 41 —— brisk
20 68 44 37 —— ditto
21 71 47 40 —— ditto
22 +77 51 46 S E ditto
23 72 53 49 —— ditto
24 72 49 46 S ditto
25 62 44 31 S W ditto 0.50
26 62 52 54 —— little 0.03
◯ 27 59 49 44 W ditto 0.47
28 60 47 43 —— ditto 0.03
29 59 42 37 —— ditto 0.15
30 60 45 40 —— ditto 0.01
31 59 42 37 —— ditto 0.02
Means 64.7 45.9 41.6 2.63

[page] 410

1820 June. Morning. Afternoon. Night.
Barometer. Hygrometer. Weather. Barometer. Hygrometer. Weather. Barometer. Hygrometer. Weather.
1 29.60 56 48 8 showers -29.60 57 -40 17 fine 29.64 51 43 8 overcast
2 29.62 56 44 12 fine 29.65 58 50 8 showers 29.71 50 47 3 fine
3 29.80 50 48 2 rain 29.84 56 52 4 rain 29.89 50 46 4 overcast
4 29.95 51 43 8 fine 29.95 51 45 6 fine 29.99 48 45 3 same
5 30.01 56 46 10 dull 30.01 66 51 15 same 29.83 59 56 3 rain
6 29.88 52 50 2 rain 30.02 56 56 rain 30.13 49 45 4 fine
7 30.08 59 47 12 overcast 30.05 61 46 15 dull 30.03 50 45 5 same
8 29.97 62 53 9 same 29.94 65 55 10 showers 29.87 58 55 3 dull
9 29.83 60 55 5 same 29.94 65 55 6 same 29.73 50 47 3 same
⊕ 10 29.77 63 47 16 same 29.79 60 41 19 same 29.75 50 45 5 very fine
11 29.66 53 43 10 same 29.62 54 52 2 rain 29.62 50 50 rain
12 29.78 58 52 6 fine 29.85 63 50 13 fine 29.93 50 47 3 very fine
13 29.94 58 51 7 same 29.91 57 56 1 thund.storm 29.96 51 51 rain
14 30.19 54 50 4 dull 30.12 59 49 10 dull 30.04 58 50 3 same
15 29.88 60 54 6 same 29.96 59 52 7 same 30.00 49 47 2 very fine
16 30.01 61 49 12 fine 29.99 67 51 16 fine 29.98 58 54 4 fine
17 30.01 62 49 13 same 30.01 67 51 16 same 30.05 58 54 4 same
18 30.11 62 46 16 overcast 30.05 67 49 18 dull 29.95 57 51 6 dull
19 29.79 63 51 12 same 29.80 63 48 15 same 29.80 53 44 9 same
20 29.71 56 51 5 rain 29.82 61 55 6 showers 29.87 54 52 2 showers
21 29.92 62 50 12 very fine 29.9 66 54 12 very fine 30.03 52 51 1 very fine
22 30.09 62 56 6 mist 30.10 75 57 18 same 30.12 60 59 1 fine
23 30.20 66 57 9 very fine 30.20 77 68 14 same 30.23 65 60 5 very fine
24 30.27 74 60 14 same 30.29 84 65 19 same 30.33 68 61 7 same
25 30.40 77 62 15 same 30.40 87 67 20 same 30.43 73 63 10 same
26 30.44 81 67 14 same 30.43 89 +70 19 same 30.44 69 61 8 same
◯ 27 30.44 79 66 13 same +30.46 83 70 13 same 30.34 69 67 2 same
28 30.33 85 67 18 same 30.24 84 64 +20 same 30.23 69 62 7 same
29 30.19 65 60 5 same 30.06 71 63 8 same 30.03 58 58 rain
30 29.99 65 62 3 showers 29.94 62 60 2 overcast 30.09 55 52 3 overcast
Means 29.995 62.2 52.8 9.4 29.994 66.2 54.5 11.6 30.001 56.2 52.2 3.8

[page] 411

1820 June. Temperature. Wind. Rain.
Max. Min. Sun. Rad. Direction. Force.

JUNE.

Mean Temperature 59.1° Weight of vapour in a cubic foot.

— Pressure. 29.996 inches

— Dew-point. 52.2° Mean. 5.315 grs.

— Force of vapour. 0.431 inch Maximum. 8.186 "

— Degree of dryness. 6.9° Minimum. 3.130 "

— Degree of moisture. 790°

Least observed degree of moisture 530

WINDS.

N. 3=48° N.E 0 E. 3=63° S.E. 4=60° S. 0 S.W. 4=56° W. 5=47° N.W. 11=50°

Amount of rain, &c. 2.11 inches

— of evaporation. 3.42 inches

REMARKS.

The firs